Hi 
Anyone who consider purchasing a new standard 4-wheel farm tractor, might have a lot of different brands and models to choose from, but when it comes to which type of engine/fuel to choose, the choice is very simple, as all new farm tractors run on diesel fuel. To my knowledge, the last gasoline fuelled tractor tested at the Nebraska Tractor Test Laboratory (NTTL), was an International 284 tested in May 1978 - more than 40 years ago [Wendel, 1985].
If on the other hand you are considering a 2-wheel tractor, some models are offered with either a gasoline or a diesel engine. There are a number of advantages and disadvantages for both types of engine/fuel, which can make it difficult to choose the one best suited, and either one can be the best choice, depending on ones individual situation.
What I find especially interesting when considering this choice, is the fact that a diesel engine with a lower power rating than a comparable gasoline engine, often is perceived to be more powerful by some dealers and users, compared to its gasoline alternative. Why is that? Is there a difference between "gasoline-hp" and "diesel-hp"? Probably not! Diesel engines also uses less fuel for the same work done as a comparable gasoline engine. Why is that? Is the Diesel-cycle more efficient than the Otto-cycle in the gasoline engine for instance? I have been looking into this interesting topic for some time now, and although I do not consider myself an expert, I hope I might be able to present some of the things that I have learned along the way, and that some of the members of this great forum will have an interest in reading it, and perhaps learn a little themselves too. I can easily have made mistakes or misunderstood some issues; in that case, I hope that members with more knowledge on this topic than I, will come forward and make us all a little wiser.
To represent the two different engine/fuel types, I have chosen the Honda GX390 gasoline engine rated at 11.7 hp (8.7 kW), and the Kohler KD440 diesel engine rated at 10.1 hp (7.4 kW), as both these engines are available on some 2-wheel tractors (like the BCS 853 from Earth Tools, and some BCS and Grillo models sold in Europe), and I have been able to find the necessary engine performance data for both engines on the Internet:
Honda GX390:
Honda Engines
Kohler KD440:
http://www.kohlerpower.in/en/manuals/engines/KD440.pdf
In order to properly describe the characteristics of an engine, a number of engine performance parameters are necessary. The two best ones, being the torque curve and the power curve. Sadly, these important torque and power characteristics are not always easy to find, and often an engine is simply marketed with its rated power only. This is the peak power that the engine is capable of delivering, when running at wide open throttle and with the maximum torque load that will allow the engine to maintain rated speed. Rated speed is the maximum engine speed that the governor will allow, and is the maximum speed, at witch the engine is designed to operate continuously.
More interesting for many tasks however, is how the engine perform in the medium to low speed range and at part-load, and that is best seen on the torque/power curves. Sadly, there have been so many different standards for measuring small engine performance over the years, that it can be difficult/impossible to compare two or more engines directly. I am not familiar with these standards, but please note that these two engines were tested using different standards (GX390 used SAE J1349, and Kohler KD440 used ISO 3046/1), which might give values that are difficult to compare directly, but the shape of the curves still represent the two different engine types well, I think.
To be able to compare the performance curves of the two engines more directly and in the same scale, I have drawn a figure of the curves on top of one-another, using the NB and MB curves for the diesel engine:
As can be seen from the 2 pair of performance curves, the gasoline engine is not just more powerful at peak power as advertised, it is more powerful than the diesel engine at any speed from peak down to its lowest recommended operating speed of 2000 rpm (see the 2 lower power curves)! This came as a surprise to me, as I expected the power curve for the diesel engine to cross the curve for the gasoline engine at some point, and get on top at lower speeds. How can a lower rated engine otherwise feel more powerful at medium to low speeds? As power is the product of torque - the twisting moment of an engine - multiplied by engine speed, and these two engines have the same speed range, it is no surprise that the gasoline engine have more torque than the diesel at any speed as well (see the 2 upper torque curves). As power is also the rate of doing work, i. e. work per unit of time, the gasoline engine will produce more work than the diesel at any given speed, so why is the diesel engine then often perceived as more powerful - especially at lower speeds?
This is due to engine elasticity I think, which is usually higher in a diesel engine than in a gasoline one. An engine is considered to be more elastic ("torquey"), if peak torque is produced at low engine speed, and increases significantly as engine speed drops below rated speed. Similarly, an engine is considered to be less elastic ("peaky"), if peak torque is produced at high engine speed, and torque only rises slightly as engine speed drops below rated speed.
As can be seen on the lower torque curve, the diesel engine has its peak torque (23.3 Nm) further to the left than the gasoline engine on the upper curve, primarily due to its higher volumetric efficiency at part-load. The torque curve for the KD440 also rises linear almost all the way from 3600 rpm down to peak torque, whereas the GX390 curve is steeper at 3600 rpm, but becomes more and more flat approaching peak torque. These curves enable the gasoline engine to cope better with increased load at high speeds (3600 rpm down to around 3000 rpm), but also to slowly run out of extra torque between around 3000 rpm down to 2500 rpm, if the load is not reduced. The diesel engine on the other hand, will lose more engine speed at high rpm as the torque curve is flatter, whereas it will be able to "lug" itself out of an overload situation approaching peak torque much better, as the torque curve rises all the way to peak. Below peak torque, the torque curve of the KD440 remains relative flat, whereas it drops relative steep for the GX390. It is probably no coincidence, that the torque and power curve for the Honda engine ends at 2000 rpm, whereas those for the Kohler continues down to 1600 rpm. I would expect the torque curve for the Honda engine to drop steeply below 2000 rpm, whereas the Kohler engine is able to produce useful torque, all the way down to 1600 rpm.
It might at first seem odd to talk about torque rise - also called torque backup, torque reserve or torque elasticity, when in fact the curves drops from peak torque to torque at rated engine speed. This is because the curves are read from right to left, i. e. from rated speed towards low idle. Torque rise is the difference between peak torque, and torque at rated speed, and indicates how well the engine will be able to handle an increased load. Torque rise is usually calculated as a percentage of the torque at rated speed.
The GX390 has a torque rise of 26.4-23.1 = 3.3 Nm (or 3.3/23.1x100 = 14.3%), whereas the KD440 has a torque rise of 23.3-19.6 = 3.7 Nm (or 3.7/19.6x100 = 18.9%).
For tractors, trucks and other heavy equipment, an elastic engine is desirable, as it better allows the engine to cope with an increased load, without the need for a downshift in gear, or the risk of stalling the engine. As the load increases, the engine speed will naturally drop, but as torque rises, an engine will only drop in speed until it reaches a stable condition, where the torque available matches the torque load. If the load increases beyond peak torque though, both engine types will stall unless a downshift is possible, as torque will drop with reduced engine speed. As peak torque is available at a lower engine speed in the KD440 engine, this engine has a better medium to low speed performance than the GX390.
At best, the power band is often the only way that engine elasticity is described. When comparing engines with the same speed range like in this example, the power band is useful when comparing the speed drop between peak torque and rated power of the two engines. These two engines both have a rated speed of 3600 rpm, and the gasoline engine produces its peak torque at 2500 rpm, the diesel at 2200 rpm. This gives the gasoline engine a narrower power band of 3600-2500 = 1100 rpm (giving an engine speed elasticity of 1100/2500x100 = 44.0%), compared to the diesel engines power band of 3600-2200 = 1400 rpm (giving an engine speed elasticity of 1400/2200x100 = 63.6%). On the other hand, the power band does not say anything about the difference in torque rise at all! If the power band is used as the only parameter to describe the elasticity of an engine, it gives a very incomplete picture. When taking the torque rise into account as well as the speed elasticity, much more information is available, and both parameters favours the diesel engine, as expected.
These two factors - the higher torque rise and the wider power band - both contributes to the higher elasticity of the diesel engine, and that again makes it more suitable for powering a tractor. By multiplying the two sorts of elasticity (the torque elasticity and the engine speed elasticity), an overall engine elasticity - the power index - is the result, as it combines the torque rise and the power band into a single value. The GX390 has a power index of 1.143x1.440 = 1.646 or 64.6%, and the KD440 has a power index of 1.189x1.636 = 1.945 or 94.5%. This gives the Kohler engine an engine elasticity which is (1.945-1.646)/1.646x100 = 18.2% higher than that of the Honda engine! [Floessel 1950; Lane 1966].
Although in this comparison the gasoline engine is the most powerful one with 11.7 hp compared to 10.1 hp for the diesel engine, the more elastic characteristic of the diesel engine (94.5% compared to 64.6%) might give the impression that it is more powerful when using it in real life in a tractor, as it copes better with an increased load; especially if the engines are not run at wide open throttle and at rated speed. In reality, the KD440 having "only" 10.1 hp, is of course less powerful than the GX390 having 11.7 hp, but its much higher elasticity makes it more suited for work in the medium to low-speed range and at part-load, where it will probably spend most of its time.
In order to see why the diesel engine is more fuel efficient than the gasoline one, we have to look at 2 of the main differences between the two engine types; the compression ratio and the volumetric efficiency.
Since the early days of internal combustion engines, engineers have known that a high compression ration was the key to increasing the thermal efficiency of an engine. Due to the risk of knocking at high combustion chamber temperatures, the GX390 engine has been designed with a compression ratio of only 8.2:1, enabling the engine to run on regular grade gasoline with a Pump Octane Number of 86 PON or higher in for instance the US and Canada, or a Research Octane Number of 91 RON or higher, as sold in most of Europe and in Australia and New Zealand. As there is no fuel in the intake air of a diesel engine, there is also no risk of premature auto-ignition. A diesel engine need a compression ratio of around 14:1 or greater to auto-ignite, and the KD440 has been designed with a compression ratio of 20.3:1, which is 20.3/8.2 = 2.48 times that of the Honda engine!
The thermal efficiency of an engine is a measure of the amount of stored chemical energy contained in the fuel, that is converted to mechanical energy through combustion. In other words, it is a measure of how efficient the engine uses the fuel. The thermodynamics involved in an internal combustion engine are very complex, so often the cycles are split up and simplified, to make them easier to handle. With these simplifications, an engine is often called theoretical or ideal, which basically mean, that all processes are "best case scenarios" that follows the basic laws of thermodynamics. In an ideal engine, there is no friction for instance, and there is no heat lost to the surroundings through cooling or hot exhaust. As we all know, there are friction and heat losses in a real engine, and the efficiencies of an ideal engine will therefore always be higher as might be expected in a real engine.
As seen on the figure below, the Diesel-cycle has a lower theoretical thermal efficiency than the Otto-cycle, if they have the same compression ratio! In the real world though, the diesel engine has a much higher compression ration - as seen above, and therefore a higher theoretical thermal efficiency than the gasoline one, leading to a lower fuel consumption. [Goering & Hansen, 2008]
Sadly I have not been able to find much official information on the fuel consumption of the GX390 engine, but Honda states that it uses 3.5 L/h at 3600 rpm at continuous rated power:
http://www.honda-engines-eu.com/documents/10912/16004/TS_GX390
In order to compare the fuel consumption of engines, it is not fair to simply look at fuel consumption itself, without taking the amount of power produced into account. This is exactly what the term specific fuel consumption does, as it is fuel consumption divided by power.
Kohler provides a specific fuel consumption curve on the page that I linked to at the top. The curve shows a fuel consumption of around 257 g/kWh at rated speed. With an average density of 0.835 kg/L, that is equal to 257/0.835/1000 = 0.3078 L/kWh. [Liljedahl et al., 1989]
The Honda engine produces 8.7 kW using 3.5 L/h, which gives a specific fuel consumption of 3.5/8.7 = 0.4023 L/kWh.
This means, that with both engines running at wide open throttle and producing rated power, the diesel engine consumes (0.4023-0.3078)/0.4023x100 = 23.5% less fuel by volume than the gasoline engine for the same amount of power produced. Or in other words, it uses a little more than 3/4 the amount of fuel compared to the gasoline one, partly due to its higher thermal efficiency. To be fair to the gasoline engine, part of the fuel saving on the diesel engine in this calculation, comes from the fact that diesel fuel has a higher energy content (a higher heating value) per unit of volume than gasoline, due to its higher density. By using a gasoline density of 0.733 kg/L, the Honda engine has a fuel consumption based on mass, of 0.4023x0.733x1000 = 294.9 g/kWh. This means, that the Kohler engine uses (294.9-257)/294.9x100 = 12.9% less fuel than the Honda, when the fuel consumption is based on mass.
I have not been able to find any fuel consumption data for the medium to low speed range and/or for part-loads for these 2 engines, but due to their higher volumetric efficiency, diesel engines become more and more fuel efficient as speed and load decreases, compared to the gasoline ones. Both types of engine however, are most fuel efficient at rated speed and load, and are becoming less and less efficient, as speed and load is reduced. As this trend is more pronounced by the gasoline engine however, its fuel efficiency is decreasing much faster than for the diesel engine.
The term volumetric efficiency is used to describe how efficient an engine is at filling the combustion chamber with air, or air/fuel mixture, during the intake stroke. Or in other words, it describes the pumping losses through the air filter, carburettor, throttle valve, intake manifold and past the intake valve, that the air has to overcome before reaching the combustion chamber. As the piston moves down during the intake stroke on a naturally aspirated engine, it creates a partial vacuum in the combustion chamber, which the outside atmospheric pressure will try to equalise. Due to the inertia of the incoming air, the pumping losses, and the short time that the intake valve is open, the combustion chamber will not be filled completely though, thereby limiting the amount of air or air/fuel that is available during the power stroke. As pumping losses increases rapidly with increasing engine speed, less and less air is available for combustion, limiting the amount of fuel that can be burnt during the power stroke, thereby also limiting the maximum power output of the engine. Both types of engine simply runs out of air as they approaches their maximum speed.
Another important difference between the gasoline and the diesel engine, is the way in which the air and fuel intake is regulated. The diesel engine has no throttle in its air intake, which means, that in each intake stroke, the combustion chamber will hold its maximum amount of air. The fuel pump will regulate the amount of fuel needed, depending on load and speed. In the gasoline engine on the other hand, the air/fuel mixture is throttled, when the engine is working at reduced load or speed.
This important difference is of little importance when the engines are running at wide open throttle and at rated speed and power. In that case, the diesel pump will supply its engine with the maximum amount of fuel possible, based on the air available. Similarly in the gasoline engine, where the maximum amount of air/fuel mixture is available to the engine.
If the load and speed is reduced though, the two engines reacts very differently. In the diesel engine, the same amount of air is available, whereas the fuel pump will reduce the amount of fuel, to fit the engines need. The engine will burn very lean and fuel efficient, and produce high torque values, as the large amount of air in the combustion chamber, will produce a high pressure on the piston. In the gasoline engine on the other hand, the throttle valve will reduce not only the amount of fuel to the engine, but also the amount of air! This leads to a much lower pressure in the combustion chamber and on the piston, and thus to lower torque values. On top that, the partly closed throttle valve will increase the pumping losses dramatically, and thereby further reduce the already lower volumetric efficiency of the gasoline engine.
There are not many data available for small engines, but the figure below illustrates this issue for farm tractors tested at NTTL [Liljedahl et al., 1989]:
Note that the unit for fuel efficiency in the figure above is the inverse of the one used for the GX390 and KD440 in my examples above! At 100% load, the average gasoline tractor produced around 2.1 kWh/L in 1976, and the diesel tractors tested in 1984 around 3.0 kWh/L - or (3.0-2.1)/2.1x100 = 42.9% more for the same amount of fuel. At 21.25% load, the numbers are around 0.9 kWh/L for the gasoline tractors and 1.9 kWh/L for the diesel tractors. Now the diesel engine produces (1.9-0.9)/0.9x100 = 111.1% more - or more than twice the amount of work for the same amount of fuel!!! In other words, the diesel engine consumes 0.9/1.9 = 0.47 - or less than 1/2 that of the gasoline one. As expected, both tractor types are less efficient at reduced load - the gasoline ones are just more than twice as bad as the diesel. The same trend can be expected for the Honda and Kohler engines.
If, for comparison, we take the inverse/reciprocal of the specific fuel consumption for the GX390 and KD440 engines, we get 1/0.4023 = 2.5 kWh/L, and 1/0.3078 = 3.2 kWh/L respectively, which is better than for the farm tractors above for both types of fuel! As specific fuel consumption tends to increase with increasing engine size, this indicates that both these engines are very fuel efficient, although they are much smaller than the average farm tractor engines of 1976 and 1984.
To summerise:
Although the diesel engine in this comparison has a lower power output than the gasoline engine all the way from peak power down to 2000 rpm, its 18.2% higher engine elasticity makes it feel more powerful. The diesel engines better volumetric efficiency - especially at part-load - is the main reason for its higher elasticity, as it gives the engine a higher torque rise as well as a wider power band compared to the gasoline engine.
Due to its much higher compression ratio, the diesel engine also has a higher thermal efficiency, which leads to a lower fuel consumption. This advantage becomes even bigger at part-load, where the diesel engine has a much higher volumetric efficiency than the gasoline one.
No wonder, that diesel engines have become so popular in almost all heavy-duty applications.
Best regards
Jens
Anyone who consider purchasing a new standard 4-wheel farm tractor, might have a lot of different brands and models to choose from, but when it comes to which type of engine/fuel to choose, the choice is very simple, as all new farm tractors run on diesel fuel. To my knowledge, the last gasoline fuelled tractor tested at the Nebraska Tractor Test Laboratory (NTTL), was an International 284 tested in May 1978 - more than 40 years ago [Wendel, 1985].
If on the other hand you are considering a 2-wheel tractor, some models are offered with either a gasoline or a diesel engine. There are a number of advantages and disadvantages for both types of engine/fuel, which can make it difficult to choose the one best suited, and either one can be the best choice, depending on ones individual situation.
What I find especially interesting when considering this choice, is the fact that a diesel engine with a lower power rating than a comparable gasoline engine, often is perceived to be more powerful by some dealers and users, compared to its gasoline alternative. Why is that? Is there a difference between "gasoline-hp" and "diesel-hp"? Probably not! Diesel engines also uses less fuel for the same work done as a comparable gasoline engine. Why is that? Is the Diesel-cycle more efficient than the Otto-cycle in the gasoline engine for instance? I have been looking into this interesting topic for some time now, and although I do not consider myself an expert, I hope I might be able to present some of the things that I have learned along the way, and that some of the members of this great forum will have an interest in reading it, and perhaps learn a little themselves too. I can easily have made mistakes or misunderstood some issues; in that case, I hope that members with more knowledge on this topic than I, will come forward and make us all a little wiser.
To represent the two different engine/fuel types, I have chosen the Honda GX390 gasoline engine rated at 11.7 hp (8.7 kW), and the Kohler KD440 diesel engine rated at 10.1 hp (7.4 kW), as both these engines are available on some 2-wheel tractors (like the BCS 853 from Earth Tools, and some BCS and Grillo models sold in Europe), and I have been able to find the necessary engine performance data for both engines on the Internet:
Honda GX390:
Honda Engines
Kohler KD440:
http://www.kohlerpower.in/en/manuals/engines/KD440.pdf
In order to properly describe the characteristics of an engine, a number of engine performance parameters are necessary. The two best ones, being the torque curve and the power curve. Sadly, these important torque and power characteristics are not always easy to find, and often an engine is simply marketed with its rated power only. This is the peak power that the engine is capable of delivering, when running at wide open throttle and with the maximum torque load that will allow the engine to maintain rated speed. Rated speed is the maximum engine speed that the governor will allow, and is the maximum speed, at witch the engine is designed to operate continuously.
More interesting for many tasks however, is how the engine perform in the medium to low speed range and at part-load, and that is best seen on the torque/power curves. Sadly, there have been so many different standards for measuring small engine performance over the years, that it can be difficult/impossible to compare two or more engines directly. I am not familiar with these standards, but please note that these two engines were tested using different standards (GX390 used SAE J1349, and Kohler KD440 used ISO 3046/1), which might give values that are difficult to compare directly, but the shape of the curves still represent the two different engine types well, I think.
To be able to compare the performance curves of the two engines more directly and in the same scale, I have drawn a figure of the curves on top of one-another, using the NB and MB curves for the diesel engine:
As can be seen from the 2 pair of performance curves, the gasoline engine is not just more powerful at peak power as advertised, it is more powerful than the diesel engine at any speed from peak down to its lowest recommended operating speed of 2000 rpm (see the 2 lower power curves)! This came as a surprise to me, as I expected the power curve for the diesel engine to cross the curve for the gasoline engine at some point, and get on top at lower speeds. How can a lower rated engine otherwise feel more powerful at medium to low speeds? As power is the product of torque - the twisting moment of an engine - multiplied by engine speed, and these two engines have the same speed range, it is no surprise that the gasoline engine have more torque than the diesel at any speed as well (see the 2 upper torque curves). As power is also the rate of doing work, i. e. work per unit of time, the gasoline engine will produce more work than the diesel at any given speed, so why is the diesel engine then often perceived as more powerful - especially at lower speeds?
This is due to engine elasticity I think, which is usually higher in a diesel engine than in a gasoline one. An engine is considered to be more elastic ("torquey"), if peak torque is produced at low engine speed, and increases significantly as engine speed drops below rated speed. Similarly, an engine is considered to be less elastic ("peaky"), if peak torque is produced at high engine speed, and torque only rises slightly as engine speed drops below rated speed.
As can be seen on the lower torque curve, the diesel engine has its peak torque (23.3 Nm) further to the left than the gasoline engine on the upper curve, primarily due to its higher volumetric efficiency at part-load. The torque curve for the KD440 also rises linear almost all the way from 3600 rpm down to peak torque, whereas the GX390 curve is steeper at 3600 rpm, but becomes more and more flat approaching peak torque. These curves enable the gasoline engine to cope better with increased load at high speeds (3600 rpm down to around 3000 rpm), but also to slowly run out of extra torque between around 3000 rpm down to 2500 rpm, if the load is not reduced. The diesel engine on the other hand, will lose more engine speed at high rpm as the torque curve is flatter, whereas it will be able to "lug" itself out of an overload situation approaching peak torque much better, as the torque curve rises all the way to peak. Below peak torque, the torque curve of the KD440 remains relative flat, whereas it drops relative steep for the GX390. It is probably no coincidence, that the torque and power curve for the Honda engine ends at 2000 rpm, whereas those for the Kohler continues down to 1600 rpm. I would expect the torque curve for the Honda engine to drop steeply below 2000 rpm, whereas the Kohler engine is able to produce useful torque, all the way down to 1600 rpm.
It might at first seem odd to talk about torque rise - also called torque backup, torque reserve or torque elasticity, when in fact the curves drops from peak torque to torque at rated engine speed. This is because the curves are read from right to left, i. e. from rated speed towards low idle. Torque rise is the difference between peak torque, and torque at rated speed, and indicates how well the engine will be able to handle an increased load. Torque rise is usually calculated as a percentage of the torque at rated speed.
The GX390 has a torque rise of 26.4-23.1 = 3.3 Nm (or 3.3/23.1x100 = 14.3%), whereas the KD440 has a torque rise of 23.3-19.6 = 3.7 Nm (or 3.7/19.6x100 = 18.9%).
For tractors, trucks and other heavy equipment, an elastic engine is desirable, as it better allows the engine to cope with an increased load, without the need for a downshift in gear, or the risk of stalling the engine. As the load increases, the engine speed will naturally drop, but as torque rises, an engine will only drop in speed until it reaches a stable condition, where the torque available matches the torque load. If the load increases beyond peak torque though, both engine types will stall unless a downshift is possible, as torque will drop with reduced engine speed. As peak torque is available at a lower engine speed in the KD440 engine, this engine has a better medium to low speed performance than the GX390.
At best, the power band is often the only way that engine elasticity is described. When comparing engines with the same speed range like in this example, the power band is useful when comparing the speed drop between peak torque and rated power of the two engines. These two engines both have a rated speed of 3600 rpm, and the gasoline engine produces its peak torque at 2500 rpm, the diesel at 2200 rpm. This gives the gasoline engine a narrower power band of 3600-2500 = 1100 rpm (giving an engine speed elasticity of 1100/2500x100 = 44.0%), compared to the diesel engines power band of 3600-2200 = 1400 rpm (giving an engine speed elasticity of 1400/2200x100 = 63.6%). On the other hand, the power band does not say anything about the difference in torque rise at all! If the power band is used as the only parameter to describe the elasticity of an engine, it gives a very incomplete picture. When taking the torque rise into account as well as the speed elasticity, much more information is available, and both parameters favours the diesel engine, as expected.
These two factors - the higher torque rise and the wider power band - both contributes to the higher elasticity of the diesel engine, and that again makes it more suitable for powering a tractor. By multiplying the two sorts of elasticity (the torque elasticity and the engine speed elasticity), an overall engine elasticity - the power index - is the result, as it combines the torque rise and the power band into a single value. The GX390 has a power index of 1.143x1.440 = 1.646 or 64.6%, and the KD440 has a power index of 1.189x1.636 = 1.945 or 94.5%. This gives the Kohler engine an engine elasticity which is (1.945-1.646)/1.646x100 = 18.2% higher than that of the Honda engine! [Floessel 1950; Lane 1966].
Although in this comparison the gasoline engine is the most powerful one with 11.7 hp compared to 10.1 hp for the diesel engine, the more elastic characteristic of the diesel engine (94.5% compared to 64.6%) might give the impression that it is more powerful when using it in real life in a tractor, as it copes better with an increased load; especially if the engines are not run at wide open throttle and at rated speed. In reality, the KD440 having "only" 10.1 hp, is of course less powerful than the GX390 having 11.7 hp, but its much higher elasticity makes it more suited for work in the medium to low-speed range and at part-load, where it will probably spend most of its time.
In order to see why the diesel engine is more fuel efficient than the gasoline one, we have to look at 2 of the main differences between the two engine types; the compression ratio and the volumetric efficiency.
Since the early days of internal combustion engines, engineers have known that a high compression ration was the key to increasing the thermal efficiency of an engine. Due to the risk of knocking at high combustion chamber temperatures, the GX390 engine has been designed with a compression ratio of only 8.2:1, enabling the engine to run on regular grade gasoline with a Pump Octane Number of 86 PON or higher in for instance the US and Canada, or a Research Octane Number of 91 RON or higher, as sold in most of Europe and in Australia and New Zealand. As there is no fuel in the intake air of a diesel engine, there is also no risk of premature auto-ignition. A diesel engine need a compression ratio of around 14:1 or greater to auto-ignite, and the KD440 has been designed with a compression ratio of 20.3:1, which is 20.3/8.2 = 2.48 times that of the Honda engine!
The thermal efficiency of an engine is a measure of the amount of stored chemical energy contained in the fuel, that is converted to mechanical energy through combustion. In other words, it is a measure of how efficient the engine uses the fuel. The thermodynamics involved in an internal combustion engine are very complex, so often the cycles are split up and simplified, to make them easier to handle. With these simplifications, an engine is often called theoretical or ideal, which basically mean, that all processes are "best case scenarios" that follows the basic laws of thermodynamics. In an ideal engine, there is no friction for instance, and there is no heat lost to the surroundings through cooling or hot exhaust. As we all know, there are friction and heat losses in a real engine, and the efficiencies of an ideal engine will therefore always be higher as might be expected in a real engine.
As seen on the figure below, the Diesel-cycle has a lower theoretical thermal efficiency than the Otto-cycle, if they have the same compression ratio! In the real world though, the diesel engine has a much higher compression ration - as seen above, and therefore a higher theoretical thermal efficiency than the gasoline one, leading to a lower fuel consumption. [Goering & Hansen, 2008]
Sadly I have not been able to find much official information on the fuel consumption of the GX390 engine, but Honda states that it uses 3.5 L/h at 3600 rpm at continuous rated power:
http://www.honda-engines-eu.com/documents/10912/16004/TS_GX390
In order to compare the fuel consumption of engines, it is not fair to simply look at fuel consumption itself, without taking the amount of power produced into account. This is exactly what the term specific fuel consumption does, as it is fuel consumption divided by power.
Kohler provides a specific fuel consumption curve on the page that I linked to at the top. The curve shows a fuel consumption of around 257 g/kWh at rated speed. With an average density of 0.835 kg/L, that is equal to 257/0.835/1000 = 0.3078 L/kWh. [Liljedahl et al., 1989]
The Honda engine produces 8.7 kW using 3.5 L/h, which gives a specific fuel consumption of 3.5/8.7 = 0.4023 L/kWh.
This means, that with both engines running at wide open throttle and producing rated power, the diesel engine consumes (0.4023-0.3078)/0.4023x100 = 23.5% less fuel by volume than the gasoline engine for the same amount of power produced. Or in other words, it uses a little more than 3/4 the amount of fuel compared to the gasoline one, partly due to its higher thermal efficiency. To be fair to the gasoline engine, part of the fuel saving on the diesel engine in this calculation, comes from the fact that diesel fuel has a higher energy content (a higher heating value) per unit of volume than gasoline, due to its higher density. By using a gasoline density of 0.733 kg/L, the Honda engine has a fuel consumption based on mass, of 0.4023x0.733x1000 = 294.9 g/kWh. This means, that the Kohler engine uses (294.9-257)/294.9x100 = 12.9% less fuel than the Honda, when the fuel consumption is based on mass.
I have not been able to find any fuel consumption data for the medium to low speed range and/or for part-loads for these 2 engines, but due to their higher volumetric efficiency, diesel engines become more and more fuel efficient as speed and load decreases, compared to the gasoline ones. Both types of engine however, are most fuel efficient at rated speed and load, and are becoming less and less efficient, as speed and load is reduced. As this trend is more pronounced by the gasoline engine however, its fuel efficiency is decreasing much faster than for the diesel engine.
The term volumetric efficiency is used to describe how efficient an engine is at filling the combustion chamber with air, or air/fuel mixture, during the intake stroke. Or in other words, it describes the pumping losses through the air filter, carburettor, throttle valve, intake manifold and past the intake valve, that the air has to overcome before reaching the combustion chamber. As the piston moves down during the intake stroke on a naturally aspirated engine, it creates a partial vacuum in the combustion chamber, which the outside atmospheric pressure will try to equalise. Due to the inertia of the incoming air, the pumping losses, and the short time that the intake valve is open, the combustion chamber will not be filled completely though, thereby limiting the amount of air or air/fuel that is available during the power stroke. As pumping losses increases rapidly with increasing engine speed, less and less air is available for combustion, limiting the amount of fuel that can be burnt during the power stroke, thereby also limiting the maximum power output of the engine. Both types of engine simply runs out of air as they approaches their maximum speed.
Another important difference between the gasoline and the diesel engine, is the way in which the air and fuel intake is regulated. The diesel engine has no throttle in its air intake, which means, that in each intake stroke, the combustion chamber will hold its maximum amount of air. The fuel pump will regulate the amount of fuel needed, depending on load and speed. In the gasoline engine on the other hand, the air/fuel mixture is throttled, when the engine is working at reduced load or speed.
This important difference is of little importance when the engines are running at wide open throttle and at rated speed and power. In that case, the diesel pump will supply its engine with the maximum amount of fuel possible, based on the air available. Similarly in the gasoline engine, where the maximum amount of air/fuel mixture is available to the engine.
If the load and speed is reduced though, the two engines reacts very differently. In the diesel engine, the same amount of air is available, whereas the fuel pump will reduce the amount of fuel, to fit the engines need. The engine will burn very lean and fuel efficient, and produce high torque values, as the large amount of air in the combustion chamber, will produce a high pressure on the piston. In the gasoline engine on the other hand, the throttle valve will reduce not only the amount of fuel to the engine, but also the amount of air! This leads to a much lower pressure in the combustion chamber and on the piston, and thus to lower torque values. On top that, the partly closed throttle valve will increase the pumping losses dramatically, and thereby further reduce the already lower volumetric efficiency of the gasoline engine.
There are not many data available for small engines, but the figure below illustrates this issue for farm tractors tested at NTTL [Liljedahl et al., 1989]:
Note that the unit for fuel efficiency in the figure above is the inverse of the one used for the GX390 and KD440 in my examples above! At 100% load, the average gasoline tractor produced around 2.1 kWh/L in 1976, and the diesel tractors tested in 1984 around 3.0 kWh/L - or (3.0-2.1)/2.1x100 = 42.9% more for the same amount of fuel. At 21.25% load, the numbers are around 0.9 kWh/L for the gasoline tractors and 1.9 kWh/L for the diesel tractors. Now the diesel engine produces (1.9-0.9)/0.9x100 = 111.1% more - or more than twice the amount of work for the same amount of fuel!!! In other words, the diesel engine consumes 0.9/1.9 = 0.47 - or less than 1/2 that of the gasoline one. As expected, both tractor types are less efficient at reduced load - the gasoline ones are just more than twice as bad as the diesel. The same trend can be expected for the Honda and Kohler engines.
If, for comparison, we take the inverse/reciprocal of the specific fuel consumption for the GX390 and KD440 engines, we get 1/0.4023 = 2.5 kWh/L, and 1/0.3078 = 3.2 kWh/L respectively, which is better than for the farm tractors above for both types of fuel! As specific fuel consumption tends to increase with increasing engine size, this indicates that both these engines are very fuel efficient, although they are much smaller than the average farm tractor engines of 1976 and 1984.
To summerise:
Although the diesel engine in this comparison has a lower power output than the gasoline engine all the way from peak power down to 2000 rpm, its 18.2% higher engine elasticity makes it feel more powerful. The diesel engines better volumetric efficiency - especially at part-load - is the main reason for its higher elasticity, as it gives the engine a higher torque rise as well as a wider power band compared to the gasoline engine.
Due to its much higher compression ratio, the diesel engine also has a higher thermal efficiency, which leads to a lower fuel consumption. This advantage becomes even bigger at part-load, where the diesel engine has a much higher volumetric efficiency than the gasoline one.
No wonder, that diesel engines have become so popular in almost all heavy-duty applications.
Best regards
Jens
