Please explain this difference between diesel and gasoline engines

[etpm]

I know someone here can help me with this question. Maybe the question should be spark ignition versus compression ignition, but I will just use gasoline spark ignition vs diesel compression ignition. Anyway, gasoline engines are very sensitive to air/fuel ratios. For example, gasoline engines when run too lean can run way too hot and this can result in pre-ignition, burnt valves, and melted pistons. I have personal experience with a melted piston crown, darn it. So the air/fuel mixture is closely controlled. Diesel engines don't control the air at all, the engines are always sucking in as much air as possible. This means that the air/fuel mixture is all over the place. So why is it that a compression ignition diesel engine can tolerate the very lean condition at low throttle settings? And, from what I have read here, diesel engines run hotter at higher throttle settings and running them at too low of a throttle setting, such as excessive idling, can cause problems such as "wet stacking". The reason being that the engine is running too cool and all the fuel is not burning completely. It's just idle curiosity on my part and I figure a good answer is likely to be found here.
Thanks,
Eric

 

[big bubba]

a lengthily post. for simplicity, can you reduce your question to a sentence or two? regards

 

[dodge man]

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.

 

[CalG]

In one sentence:

It is the nature of the fuels used, not the engine mechanicals.

To elaborate, oil fuels burn more slowly regardless of how many ignition centers are produced under compression.

High volatility fuels, combust much more quickly, and often destructively.

Alcohol mixes are an attempt to "cool/slow things down" inside the combustion chamber (NOT the EPA kind)

 

[TractorTech]

Diesel engines bring in a fixed volume of air per stroke, compress it anywhere from 14:1 to as high as 25:1 and then a very precise amount of fuel is injected under very high pressure to atomize it on the piston to combust. So, “the air fuel mixture is not all over the place”.

 

[Williy]

Also diesel engines usually when taken care of last longer
than a gas engine and the diesel engine usually gets better
fuel mileage and more lugging power vs a gas job

willy

 

[CalG]

Diesel engines bring in a fixed volume of air per stroke, compress it anywhere from 14:1 to as high as 25:1 and then a very precise amount of fuel is injected under very high pressure to atomize it on the piston to combust. So, “the air fuel mixture is not all over the place”.

This is non sense!

 

[CalG]

Also diesel engines usually when taken care of last longer
than a gas engine and the diesel engine usually gets better
fuel mileage and more lugging power vs a gas job

willy

This is not the intent of the post.

I don't give greater care to my compression ignition Hurlimann, than I do to the spark fired gasoline fueled Fordson E27n. The Fordson has a rated RPM of 1100. with pto speed 540. talk of lugging....

 

[CalG]

Diesel engines bring in a fixed volume of air per stroke, compress it anywhere from 14:1 to as high as 25:1 and then a very precise amount of fuel is injected under very high pressure to atomize it on the piston to combust. So, “the air fuel mixture is not all over the place”.

Can you elaborate using the Detroit diesel in example?

 

[Anakist]

It's a combo of things. Diesel burns slower than petrol so the peak temperature isn't as high. Diesels are built heavier out of more sturdy materials. They run *very* lean compared to petrol. Diesels run slower so there is comparatively more time for the heat to disperse between ignition events.

You can still melt diesel pistons. Diesels come the other way where more fuel makes the combustion hotter. So winding up the fuel screw and hoping for the best is the same as leaning out the fuel on a petrol. Eventually you will get the piston hot enough to melt.

From a quick google, diesels run about 18 or 20 to 1 under high load, as opposed to petrol running 12.5 - 14.7 depending on your tune. Some petrol engines can run in lean burn for economy under light load but I don't know the specifics.