Please explain this difference between diesel and gasoline engines

[CalG]

Maybe in the most simplistic design IF motor stopped in worst spot it would need 2 crank revolutions for ECU to "figure it out" but anything built in this century will "know" the position in 1/4 to 1/2 of revolution no matter where it stopped.

ECUs are in a love hate relationship with time.....

 

[Grumpycat]

12.5:1 to 14.7:1 compression ratio in a petrol motor? I believe you're talking high performance sports cars and certain sports motorcycles. Most petrol engines are around the 10:1:1 to 12.5:1 ratio, even lower - unless I'm way out of date

Prius has a 13.5:1 mechanical ratio. Runs on 87 AKI.

The Prius has a CVT and electric traction motors all under drive by wire. So it can keep the throttle out of ranges harmful to the engine.

Also the ECU controls wider range intake valve timing that any other, controls the effective compression pressure by letting some of the charge burp back into the intake.

 

[Grumpycat]

Diesel fuel is about 1.5 times the energy potential in a given volume of fuel. Diesel explodes more steadily expanding to push the piston down, less of a bang that gasoline gives. More power is extracted from equal energy from a diesel. Because compression is greater, motor components must be stronger.

Diesel has about 1.1x the energy of gasoline.

Neither combust at explosion rates in internal combustion engines.

Carnot cycle teaches us how higher compression ratios produce greater thermal efficiency. That is why the Prius runs up to 13.5:1.

Prius ICE runs in the efficiency range of Diesels, but on gasoline.

 

[Grumpycat]

Put gasoline in a diesel engine and it will burn up.

 

[Anakist]

12.5:1 to 14.7:1 compression ratio in a petrol motor? I believe you're talking high performance sports cars and certain sports motorcycles. Most petrol engines are around the 10:1:1 to 12.5:1 ratio, even lower - unless I'm way out of date

Fuel / air ratio.

 

[pmsmechanic]

Maybe in the most simplistic design IF motor stopped in worst spot it would need 2 crank revolutions for ECU to "figure it out" but anything built in this century will "know" the position in 1/4 to 1/2 of revolution no matter where it stopped.

Do some reading on the subject. Gale Banks has good articles.

 

[rScotty]

The reason a gasoline engine will run hot and potentially melt pistons when run "lean" is that a diesel engine generally operators under a different portion of the "lean" spectrum. As shown below, peak flame temperature is achieved at the ideal air/fuel ratio (stoichiometric ratio), with the flame temperature being lower at both richer and leaner conditions.
View attachment 826104
In a gasoline engine, the goal mixture for maximum power is actually slightly rich. Visually this means the engine is operating somewhere on the sloped line between 0.5 and 1.0 in the above image. Adding air to the original "goal" mixture moves the air/fuel ratio closer to 1.0, which increases the flame temperature. This new condition is leaner than the original "goal" but is not actually "lean" in the absolute sense. If you continued to add air past the stoichiometric ratio the flame temperature would come back down and the engine would run cooler again on the right side of the peak.

This article from Summit Racing shows some common ratios for cruise, idle, and Wide-Open-Throttle relative to the stoichiometric ratio: Summit Racing - Air/Fuel Mixtures


Diesel engines operate on the opposite side of the curve: At idle they are way past 2.0 on the right side of the peak flame temperature with a very high air/fuel ratio. As more fuel is added, the air/fuel ratio gets richer and richer, moving the ratio closer to 2.0 and eventually 1.0 where the peak flame temperature occurs.


To summarize, they respond to becoming more rich or more lean differently because they operate on different sides of the peak flame temperature. WHY gasoline engines run slightly rich instead of designing the engine to handle the peak flame temperature is another topic, and likely has to do with engineering/control of the process, cost, emissions, and/or all of the above.

Ahhh..how nice.... we finally get an answer to the OP's original question about the relationship between lean burn and heat. I've sure learned a lot, and this thread has been fascinating in all directions. Worth the read.
rScotty

 

[danallen]

The only answer I have is diesels are built better/heavy duty. They have heavier duty components due to the high compression ratios. They are designed to tolerate the points you bring up.

Some people refer to diesel engines as "Compression ignition engines" and petrol engines are referred to as "Spark Ignition Engines".
A diesel engine initially compresses air, the process of compressing the air generates large amounts of heat, when the piston is nearing the top of its stroke
we then inject very high pressure atomized fuel into the combustion chamber, when atomized fuel is injected into the high pressure/high temperature air, combustion occurs.

Can you elaborate using the Detroit diesel in example?

Detroit Diesel is two stroke as opposed most regular diesel engine which are 4 stroke.

 

[dodge man]

A lot of diesels are indirect injection, much lower pressures, much simpler too but less power and higher emissions. Most of those are probably in the less than 25 horsepower category these days.

 

[ArlyA]

Diesel engines types used in boats. More complicated than I could imagine.

30 Types of Marine Diesel Engines Explained​

--------------------------------------------------------

Type of use​

A marine diesel engine is put to different uses onboard. Broadly, they can be used in the following two manners:

Marine propulsion engine​

When a marine diesel engine is used to turn the propeller and move the ship, it becomes a marine propulsion engine and is popularly known as the main engine. The engine can be run at various speeds as per need.

Diesel generator

A marine diesel engine is also coupled to an alternator and used to generate electric power onboard. There is usually more than one of these onboard and multiple generators may be put into use during critical operations. Such diesel generators are also known as auxiliary engines.

Marine Diesel Generator

Number of strokes​

We define a stroke as the travel of the piston from one end of the cylinder to the other end. Engine cylinder operations such as induction, compression, combustion (power stroke) and exhaust gas removal occur with the help of piston strokes. Based on the number of strokes it takes to complete one power stroke, an engine is classified as:

Two-stroke engine

In a two-stroke engine, a power stroke occurs every two strokes or one revolution of the engine crankshaft. The four operations are completed within one revolution. As a result, two-stroke engines provide a higher power-to-weight ratio, greater stability, simpler construction and are more compact compared to a four-stroke marine diesel engine.

Four-stroke engine

A four-stroke engine needs two complete revolutions of the crankshaft for one power stroke. The four aforementioned cylinder operations occur in four separate strokes in this type of engine.


Due to the fewer power strokes per revolution, these engines are more fuel efficient as the fuel has more time for complete combustion. It also requires less lubrication and is generally more durable than a two-stroke engine. All these qualities make four-stroke engines a more eco-friendly alternative.

Single-acting or double-acting

Reciprocating-type engines are further classified into single-acting and double-acting cylinder engines. In single-acting engines, power stroke occurs on one side of the piston, but it occurs on both sides in a double-acting engine.


There are two combustion spaces and a double piston is used to seal both of them. Power stroke takes place on the upward as well as downward stroke. In the case of a two-stroke double-acting engine, there are two power strokes per crankshaft revolution. Double-acting engines are not as popular as their counterpart as the lower combustion space can be difficult to seal.


A double-acting engine is different from an opposed-piston engine in which there is only one combustion space and two pistons on either side of it.

 

[CalG]

One of the world's biggest problem is the lack of understanding of just how useful double acting diesel engines are. I mean, they are in just about everything from toasters to airplanes, and the common man just has no idea how they work, or where they are installed in your typical heavy duty pickup truck.

;-)

 

[ArlyA]

Moving me away from boat power plants (which I love) here is a nice video that shows how gas and deisel engines work.

 

[rScotty]

The original OP's question which can be paraphrased as, "Why is a lean burn hotter?" - shows that sometimes the difficulty putting an answer together is partly because of how the question is worded.

In post # 73, Rancher Ed shows a "fuel versus air" temperature & burn chart that explains that both lean and rich burns are actually cooler. We only get max heat when the ratio of fuel to air is perfect - which is when all the fuel is burned by all the air - i.e. a perfect stoichiometric ratio

But since engines are normally designed to run a bit rich, when we start adding more air we may think we are burning lean, but what is actually a happening is the temperature goes up because we are moving our rich mix closer to a perfect air/fuel burn ratio. Once we pass that perfect ratio and really do start burning lean the temperatures drop again.

rScotty

 

[Riding a Relic]

Your statement above makes some sense. I'm gonna need to think about it and learn about how diesel burns compared to gasoline. I think the way the fuels burn is the reason, or main reason, for the differences in temperature. You also stated that when the diesel is injected it starts to burn when the right (stoichiometric) ratio is achieved. I think you are partly mistaken. I think the fuel starts to burn as soon as it can and doesn't care about the stoichiometric ratio. I still am trying to learn why the very lean burn condition of a diesel engine doesn't lead to the very hot conditions that very lean burning gasoline engines experience. And just to be clear, I am not talking about diesel engines with electronic controls, but instead diesel engines with mechanical injection. Like the engine in my 43 year old Yanmar. As near as I can tell even gasoline engines with fuel injection still burn too hot when running too lean.
Thanks,
Eric

All engines will run hot if running lean. I've seen holes burned in pistons where an injector had some of the holes stopped up. The properties of gasoline and diesel fuel are engineered to help cool and lube the cylinders.

 

[rScotty]

All engines will run hot if running lean. I've seen holes burned in pistons where an injector had some of the holes stopped up. The properties of gasoline and diesel fuel are engineered to help cool and lube the cylinders.

I believe it will run cooler both when running lean or rich. Maximum heat happens when it burns at a perfect stoichiometric ratio of fuel to air.
But most engines are set up to burn on the rich side of a perfect mix. So when more air is added it seems like lean burn is the culprit - but actually the mix is getting closer to stoichiometric.
At least that is my understanding from several posts above.
rScotty

 

[D'oh]

The Original Poster answered his own question by stating Air Flow Restricted on a Gasoline and non restricted on a Diesel.

So a Lean gasoline engine that is still allowing more air in runs Hot. A lean Diesel without any air restriction, simply slows down.

Or oppositely, too much fuel, causes a Run Away engine. While too much Fuel in a Airway Restricted Gasoline Engine causes flooding

 

[Riding a Relic]

The reason a gasoline engine will run hot and potentially melt pistons when run "lean" is that a diesel engine generally operators under a different portion of the "lean" spectrum. As shown below, peak flame temperature is achieved at the ideal air/fuel ratio (stoichiometric ratio), with the flame temperature being lower at both richer and leaner conditions.
View attachment 826104
In a gasoline engine, the goal mixture for maximum power is actually slightly rich. Visually this means the engine is operating somewhere on the sloped line between 0.5 and 1.0 in the above image. Adding air to the original "goal" mixture moves the air/fuel ratio closer to 1.0, which increases the flame temperature. This new condition is leaner than the original "goal" but is not actually "lean" in the absolute sense. If you continued to add air past the stoichiometric ratio the flame temperature would come back down and the engine would run cooler again on the right side of the peak.

This article from Summit Racing shows some common ratios for cruise, idle, and Wide-Open-Throttle relative to the stoichiometric ratio: Summit Racing - Air/Fuel Mixtures


Diesel engines operate on the opposite side of the curve: At idle they are way past 2.0 on the right side of the peak flame temperature with a very high air/fuel ratio. As more fuel is added, the air/fuel ratio gets richer and richer, moving the ratio closer to 2.0 and eventually 1.0 where the peak flame temperature occurs.


To summarize, they respond to becoming more rich or more lean differently because they operate on different sides of the peak flame temperature. WHY gasoline engines run slightly rich instead of designing the engine to handle the peak flame temperature is another topic, and likely has to do with engineering/control of the process, cost, emissions, and/or all of the above.

I've seen holes in diesel pistons due to clogged injector holes. Overheating the top of the piston. Carbon can build to a point to where it will stop up an injector hole.

 

[Portuguese Farmer]

Gas engine has to work, diesel engine likes to work.

 

[dfkrug]

Your statement above makes some sense. I'm gonna need to think about it and learn about how diesel burns compared to gasoline. I think the way the fuels burn is the reason, or main reason, for the differences in temperature. You also stated that when the diesel is injected it starts to burn when the right (stoichiometric) ratio is achieved. I think you are partly mistaken. I think the fuel starts to burn as soon as it can and doesn't care about the stoichiometric ratio. I still am trying to learn why the very lean burn condition of a diesel engine doesn't lead to the very hot conditions that very lean burning gasoline engines experience. And just to be clear, I am not talking about diesel engines with electronic controls, but instead diesel engines with mechanical injection. Like the engine in my 43 year old Yanmar. As near as I can tell even gasoline engines with fuel injection still burn too hot when running too lean.
Thanks,
Eric

You know that your old Yanmar engine is probably port injection (not DI), right? As those of us who have many JDs with the Yanmar port injection and heated air intake engines. Relatively low compression vs. DI engines, too.

Combustion speed is what I have also read differentiates lean burning issues with Diesel vs.Otto. Atomization vs. vaporization also applies...

 

[DarkBlack]

As those of us who have many JDs with the Yanmar port injection and heated air intake engines. Relatively low compression vs. DI engines, too.

Looks like you have that backwards. Direct injection diesel, tends to have a lower compression ratio, not higher