Engine Horsepower: Rule of Thumb

[duane]

People like rules of thumb to help evaluate things on an apple to apple basis. A common question is the comparison of diesel versus gas engines in a tractor. The common seat of the pants perception is that a diesel engine provides more useful power in tractor work than a gas engine. I think we all agree that is true, but how much?

First, why does a diesel have more useful power. Power, or actually horsepower is made up of two parameters, RPM and torque. With diesel horsepower, the torque parameter makes up a slightly larger component of the horsepower number. I believe that another advantage a diesel has with regard to useful power is the greater spinning mass. It acts as a flywheel with stored energy to better absorb changes in load. In addition, a diesel typically has a more efficient cooling system (liquid cooled) versus (air cooling) for a gas engine. This allows a diesel to output on a continuous basis closer to its labeled horsepower than an air cooled gas engine.

After a lot of reading and number crunching, here is my rule of thumb for shaft equivalent output over the entire power range versus the advertised horsepower. (although not included, a diesel powered tractor normally has a shaft drive vs. v-belt means of transferring the power, which can give it another 5% advantage)

10 HP electric variable speed motor =
15 Hp hydraulic motor =
19 Hp diesel =
22 Hp liquid cooled gas =
25 Hp air cooled gas

What do you think? Do you think I am way off base. What is your rule of thumb, and what it is based on.

 

[To20Chris]

The key phrase is "useful power". Technically, the dyno does not care what your engine runs on, so a 20hp diesel is the same as a 20hp engine that runs on earwax. Engines are usually rated at the peak of power, which tells you nothing about how much they produce and any other rpm. What makes a diesl have more "useful power" is the shape of the hp and torque curves. Diesels usually have a broad flat torque curve, with a low rpm peak, and thus can keep pulling as the rpms drop, which is more useful than an engine that has a high peak. Some of this is due to tuning (cam, etc.), and some is inherent to diesels.

 

[Soundguy]

</font><font color="blue" class="small">( believe that another advantage a diesel has with regard to useful power is the greater spinning mass. It acts as a flywheel with stored energy )</font>

This will be the same concept with a gas engine.. rotating metal is rotating metal.. Like the other poster said.. it doesn't matter if one is burning diesel and the other earwax

</font><font color="blue" class="small">( a diesel typically has a more efficient cooling system (liquid cooled) versus (air cooling) for a gas engine. )</font>

Um.... You do realize that there are liquid cooled gas engines.. right? /forums/images/graemlins/shocked.gif

Soundguy

 

[Dargo]

</font><font color="blue" class="small">( This will be the same concept with a gas engine.. rotating metal is rotating metal.. Like the other poster said.. it doesn't matter if one is burning diesel and the other earwax)</font>

Not to argue the point, as you are obviously correct, but perhaps the diesel would have a little more weight that is in motion. Isn't it Newton's law, er, or is it Einstein's, that an object in motion will remain in motion until acted upon by an equal and opposite force? Ah, I've slept a couple times since physics in college. Anyway, if there is more mass in motion, it will take a larger "equal" force to slow or stop that mass. Maybe? /forums/images/graemlins/confused.gif

 

[Soundguy]

Technically yes.. but here are my observations. Two engines.. one gas, and one diesel.. of equal HP.. The ones I've seen.. the gas engine is bigger than the diesel.. thus more mass. Also.. rpm is a factor. Most all the gas engines I've seen spin at a higher rpm than the diesel engines. Add those two together and you have a larger gas engine spinning at a higher rpm than a smaller diesel ( less mass ) spinning slower ( lower rpm ).

If anything.. I'd say that a diesel is more efficient at making hp per unit of fuel than a gas engine.

On a load, it takes the gas aengines higher rpms and less torque to overcome a load.. the diesel lugs thru with the higher torque and looses less rpms when it hits a load it seems. End result is about the same. again.. I'd think 2 engines that both dyno'ed the same.. are for allintents and purposes turing out the same power. I'd guess that the gas engine would be less efficient.. and possible not last as long as the diesel, as it is 'working harder' ( more rpms.. etc ) to make that saem hp that the diesel is putitng out while chugging along at a much lower rpm...


I'm not saying your wrong.. just what I've observed.

As an example.. the gas engine in one of my olg tractors is 119ci.. and makes as much hp as a NH unit that has only 93ci.. The gas engine is qhysically larger .. has more metal.. etc.

Soundguy

 

[duane]

The shape of the Torque curve vs RPM is the point. /forums/images/graemlins/smile.gif Thats why I believe a 10 HP electric engine is equal to a 19 Hp diesel. Electric motors make max torque at 0 RPM. Same concept for hydraulic motors etc <font color="yellow"> </font>

Checking to see if others have similiar or different comparison numbers for useful power equivalents.

 

[Slamfire]

Since diesel oil contains more energy than gasoline per volume unit, the gas engine has to spin faster and burn more fuel to produce the same power as a like size diesel. Given the same displacement I doubt gas engines weigh more than the diesel.

 

[keeney]

Its all about Torque (at the RPM in question).

One of the reasons for the perception is that for most applications, you are not running at the RPM for peak HP, certainly not all the time. Peak HP is usually at an RPM up near the engines max RPM. A flat or inverted torque curve will hold up better at the lower RPM's (for example when lugging down).

What is really being perceived is the torque of the engine over the range of RPM's used.

When comparing, it will be useful to scale each engine's RPM range to account for the varying peak RPM points and/or the typical final drive ratios used.

So at some point on the torque curve, say 1/3 of Peak RPM, the 10HP electric has the same or more torque than the 25HP gasoline engine being lugged down to 1/3 of its Peak RPM.


Cooling Matters for Sustained Power Output.

As far as cooling goes, the crude rule of thumb for most internal cumbustion engines is that from the energy released by burning the fuel, only 1/3 of it goes out the driveshaft as mechanical energy (gross HP). Another 1/3 goes out the tailpipe as noise, pressure, and hot gas flow. The final 1/3 goes out the cooling system.

So, if you want a sustained 50HP gross output, you will need a 50HP exhaust system and a 50HP cooling system.

It is interesting to note that most automotive engines are significatly under-cooled. This is because you rarely run your car (light truck) at max HP for any prolonged amount of time. If you do, it over-heats. For example, when towing something so heavy, or up hill, that you have the gas peddle to the floor the whole way.

Tractors, on the other hand, are designed to run near full power for prolonged periods of time. A 50 HP tractor would likely have a similar radiator size as is used in a light truck with a 250 HP engine. Both could run at 50 Hp all day without overheating. The light truck could have short bursts of peak output at 250 HP, but would overheat if run for prolonged periods at full power.

Air-cooled engines tend to skew this ratio by over-fueling. A significant amount heat is absorbed in evaporating (boiling) the un-burned or partially-burned fuel. These hot gasses exit the tailpipe as evaporated hydrocarbon emmissions. The actual air-cooling fins would typically amount for a small percentage of the cooling of the engine, even if a fan is included. So for fuel efficiency's sake, air-cooled is a poor choice. It is an OK choice if you have enough air flow inherent in the application that the cooling fins work well without a huge fan or running so rich. For example, in aircraft, or perhaps motorcycles driven on the highway (but not in stop-and-go traffic, or off-road hill-climbing, etc).

Air-cooling is also great for small size, light weight, and cheap to build (less parts).

- Rick

 

[BigEddy]

This debate has gone on many times, and will be repeated I'm sure.

Here are my thoughts.

What it comes down to is how the engine / motor reacts to loads that exceed the torque available at any given speed.

With a high rpm gas engine with a narrow power band, if the load exceeds the output of the engine, the rpms have to drop off drastically until the load reduces itself due to lower blade speed (or whatever) to match the torque output of the engine at that speed.

With a typical low rpm diesel with a flat torque curve, if the torque load increases beyond the engine's limit, the rpm drop is not as substantial, and therefore the engine powers through the load better.

With some electric motors, the torque curve is actually a negative slope, so if the load exceeds the torque available at a given speed and slows the motor, then the torque available actually increases with the rpm drop, which minimizes the overall rpm drop. The electric motor does not have to rely on the load dropping off as the speed drops.

Flywheel size, cam design, cylinder qty and size all contribute to the torque curve of an engine. Bigger flywheels and bigger cylinders result in more torque and "usually" lower rpm engines. Lots of small cylinders and a light flywheel gives a high revving engine with a narrow power band. Depending on what you want your engine to do - both are good.

Tractors and mowers are typically run at a constant rpm, with relatively constant loads. Flat torque curves, heavy flywheels and lower rpms are good for those tasks. Motocross requires quick acceleration, has highly variable loading, making tweaky 2-strokes better suited. A 40HP motocrosser is great, but try racing with a 40hp Yanmar!

 

[Peter_H]

Hi guys,

Slamfire said,
"Since diesel oil contains more energy than gasoline per volume unit, the gas engine has to spin faster and burn more fuel to produce the same power as a like size diesel. Given the same displacement I doubt gas engines weigh more than the diesel."

No, I don't think so. Diesel fuel does have more BTU's per gallon, but I believe that for a given displacement, that a smaller quantity of diesel fuel is injected.

In the period of vintage tractors, many manufacturers offered both diesel and gasoline models. Often, these were built from the same block, crank and pistons in both models. The difference was in the head. (There were some differences in the bottom end, mostly concerning supporting a distributer or injection pumps.) In these cases, the gasoline engine regularly provided slightly more usable horsepower than the diesel. In the case of the Oliver 550 the numbers are close to (forgive me, I'm citing from memory) 41 PTO HP and 35 drawbar HP for the gas and 39.5 PTO and 34 drawbar for the diesel. These are the result of the Nebraska tractor test and represent actual demonstrated usable power.

I think most of the differences in torque between diesel and gasoline engines is based on design parameters and not the fuel source.

I will now duck!

Peter
Southern Maine