Please explain this difference between diesel and gasoline engines

[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