Please explain this difference between diesel and gasoline engines

[Snobdds]

I'm sorry, I don't understand your response. Air is certainly a gas but it is the gasoline or diesel that is burning, the air is just the oxidizer. And air is being used in both engines. When the gasoline is first introduced into the cylinder it is a fine mist and/or vapor. The diesel is a fine mist. I don't know but I imagine that some of both the gasoline and the diesel turn into a gas during the combustion process. Does this phase change in a diesel engine cool the air and diesel charge in the cylinder so much that running lean doesn't lead to high temperatures like it would in a gasoline engine?
Thanks,
Eric

I think you're not understanding the physics here. It's simple, if your have more air in a combustion chamber than fuel, it's going to burn hotter due to the higher percentage of a gas (oxygen) than a fluid. The opposite of supplying more fuel than air will make an engine run cooler.

The fluid, gas or diesel, can only be atomized by air introduced into a combustion chamber. It never turns into a gas or vapor.

If you want to understand it better, dive into the 2 stroke motor and how it works. People understood how the interaction of air and fuel upon a combustion chamber were linked going back to the 50's. In a 2 stroke, if you run lean...you burn up the piston and cylinder. So having the proper air/fuel mixture is critical. You quickly learn that too much air is what drives the heat.

In a diesel, the fuel delivery is handled by a vane pump in the older mechanical set ups. It runs off a lobe on the cam that drives the pump faster as the RPMs increase. It then delivers more fuel. So there is a direct relationship to always deliver the proper amount of fuel. In the common rail system, it uses throttle position or drive by wire, to tell the ECU to dump more fuel depending on load.

Diesels can run lean, but if your pump or sensors are working, it will never run lean. Also it's critical in a diesel to have good flow of diesel to make sure it dosen't run lean. That is why there are two fuel filters to make sure good fuel delivery. Diesels do however need heat for a proper combustion, to a point. If a diesel has a high duty cycle, meaning they are running full load for hours on end, then they will need a pretty robust cooling system. Like in my powerstroke, Ford had to use two radiators to increase the duty cycle to keep the heat in check. Otherwise the motor will derate (fuel being held back) and hold back the rpms to save the engine from overheating.

So again, study the two stroke motor and it will all make sense...

 

[Eagle1]

In diesel the air is compressed and that created heat (typically above 1000ºF [538ºC]) THEN the fuel is injected just before TDC and it ignites when it contract the HOT compressed air.
In gas the fuel is mixed with air BEFORE compression, the mixture is compressed and the spark ignites it.
Gas has to be a vapor before it burns well. Diesel just uses the amount of O2 required to burn and the rest is sent on its way. Therefore more diesel more boom.

 

[Snobdds]

In diesel the air is compressed and that created heat (typically above 1000ºF [538ºC]) THEN the fuel is injected just before TDC and it ignites when it contract the HOT compressed air.
In gas the fuel is mixed with air BEFORE compression, the mixture is compressed and the spark ignites it.
Gas has to be a vapor before it burns well. Diesel just uses the amount of O2 required to burn and the rest is sent on its way. Therefore more diesel more boom.

Google...Direct injection in a gas motor.

Report back.

 

[Eagle1]

Google...Direct injection in a gas motor.

Report back.

I believe Direct Injected Gas motor introduce the gas before the compression is near completion. But I've been wrong before and will be again

Beside tech moves to fast for me, these days.

 

[Snobdds]

I believe Direct Injected Gas motor introduce the gas before the compression is near completion. But I've been wrong before and will be again

Beside tech moves to fast for me, these days.

Of course it's introduced before compression, how else would it work.

How does DI vaporize the fuel, in the air, before compression, before the fire...as you asserted to?

 

[Eagle1]

Conceptually, diesel engines operate by compressing air to high pressure/temperature and then injecting a small amount of fuel into this hot compressed air. The high temperature causes the small amount of highly atomized injected fuel to evaporate.

A Beginner’s Guide To Understanding a Diesel Engine

 

[CalG]

While one is at it,

Use Duck Duck Go to search for "Detonation", "Pre-Ignition" , "auto-ignition", "Pinging", "Flame propagation", "Stoichiometry", "anti-knock'", "octane", & "cetane"

Every fuel has it's nature.

Even producer gas........

 

[Snobdds]

Conceptually, diesel engines operate by compressing air to high pressure/temperature and then injecting a small amount of fuel into this hot compressed air. The high temperature causes the small amount of highly atomized injected fuel to evaporate.

A Beginner’s Guide To Understanding a Diesel Engine

Some of us are past the a beginners guide of understanding here...

 

[Eagle1]

Some of us are past the a beginners guide of understanding here...

Well then why ask me? Like I said I've been wrong often and don't mind learning. Not an expert in any way shape or form.

 

[Snobdds]

Well then why ask me? Like I said I've been wrong often and don't mind learning. Not an expert in any way shape or form.

I never asked you.

You came in with incorrect info and then threw a beginners guide at me.

Fuel does not vaporize, the word your looking for is atomization.

 

[Eagle1]

I never asked you.

You came in with incorrect info and then threw a beginners guide at me.

Fuel does not vaporize, the word your looking for is atomization.

Sorry wrong term and you are correct.

 

[CalG]

A non beginner's guide

https://web.mit.edu/2.61/www/Lecture%20notes/Lec.%2015%20Diesel%20engine%20characteristics.pdf

With lots of pictures! ;-)

And lots of Vaporization ;-)

 

[hosspuller]

The way I see the difference is ignition. Gasoline engines require the perfect ratio of fuel/air to ignite the charge in the cylinder. Too much or too little... no go. (see LEL & UEL) Within these limits... If the charge is lean then the burning temp is high enough to burn the top of the piston. Too much gas and you've "flooded" the engine or fuel economy is terrible. As for direct injected gasoline engines, the injector is putting the fuel in the intake air stream before or while compressing... Throttle body, intake manifold, stratified charge, etc. ("Perfect fuel/air ratio is still required for ignition.) At ignition the flame has to be controlled or one gets "Knock" Explosion vs burning. High octane (measure of burn characteristic) and ignition timing control the knock.
A diesel on the other hand is always lots of air. The more air the more fuel that be injected to burn. The fuel is broken up into little drops that burn in the hot air. "Rolling coal" is more fuel than can be burned completely. Modern controls prevent this.

To answer another poster ... Diesel engines last longer mostly because they run slower for a given output.

 

[Snobdds]

... Diesel engines last longer mostly because they run slower for a given output.

Diesels have a bigger bore and longer stroke, meaning more friction. More friction = more wear.

Diesels last longer due to the lubricating properties of their fuel. Diesel is an oil and it lubricates and cleans the top end of the combustion chamber while piston squinters lube and cool the bottom of the combustion chamber. Gas has drying properties that wash the oil film off the cylinder, creating wear.

Diesels are direct injection and they do not suffer the carbon deposits that Direct injected gas motors suffer from. Same process for gas and diesel, but very different outcomes.

With the change to low sulfur diesel, the lubricating properties have been diminished. Couple that with emissions and modern diesels will not last as long as old 2 stokes or even old HEUI with old chemistry diesel fuel.

 

[m7040]

Always been around diesel engines. Dad's truck had 6 cylinder Mercedes Benz diesel with pre chamber injection. Worked on farm tractors with two stroke big bore engines using torch to preheat with. Worked on stationary diesel engines in engineering lab to calculate performance. Now still have 6 cylinder Mercedes diesel engine in van and high hp diesel in pickup as well as Kubota diesels etc

 

[Williy]

CalG Gasoline engines don't need spark ignition to burn,

Then why do gas engines have spark plugs???????????

willy

 

[90cummins]

I didn’t read the posts but I’ll give you my explanation on gas verses diesel.
A gasoline engine works with a constant cylinder volume decreasing pressure combustion process while a diesel is constant pressure increasing volume process.
A gasoline engine takes in a fixed volume of fuel & air that does not change during combustion cycle. It is compressed ignited and peak cylinder pressure occurs after ignition near top dead center (tdc) where there is very little mechanical advantage because of the rod crankshaft relationship. As the piston goes down the cylinder the mechanical leverage increases but the cylinder pressure is also dropping so torque quickly drops off.
A diesel on the other hand takes in excess air and compresses it to very high pressures causing the air temperature to increase to the point that when diesel fuel is injected it spontaneously ignites. As the piston is pushed down the cylinder fuel continues to be injected increasing the volume and maintaining combustion pressure. Fuel continues to be injected as the rod/crankshaft approaches the point of greatest leverage. But we’re still not done here. Fuel can continue to be injected maintaining pressure on the piston until past the point of maximum leverage and the available oxygen is used up.
That’s why diesels make so much more torque and pull so hard and feel like they don’t want to quit. Diesels produce increased amounts of torque thru more degrees of crankshaft rotation than gasoline engines.
Diesels take advantage of the mechanical leverage where (gas jobs) just peter out.
Hope this helps.
90cummins

 

[Hay Dude]

Speaking of Diesel engines:

Did you know “Diesel” should always be spelled with a capital “D”?
The inventor of the Diesel engine was Rudolph Diesel.

Also, did you know Rudolph Diesel, who was almost as significant as Elon Musk back in the early 1900’s disappeared mysteriously from a passenger ship and was never found? His Diesel engine technology breakthrough was taking the world by storm. Rumor has it he was sharing his diesel technology with the British Empire to install in Royal navy submarines.

Many think he was murdered as he was costing billionaire moguls who produced coal fired ship engines their livelihoods as the diesel engine could produce much more power and for much less money. Or perhaps Der Kommisar from Germany wanted him gone for sharing diesel technology to Germany’s chief naval rival, the British.
Back then, it took many men in dangerous conditions to keep large coal fired ship engines running. Those men on the ships, sometimes as many as 200, had to housed, fed and paid to shovel coal and keep the engines running. Diesel engines made more power and took a smaller crew to keep running.

Diesel was taking over and still to this day 100 years later is still the go-to power source for most heavy tasks.

Rudolph Diesel is a very under rated inventor who changed the world forever with his Diesel engine technology and it’s still going strong today.

 

[ericm979]

First, "The High Speed Internal Combustion Engine" by Ricardo 1923 IS a MUST READ!

Last time I looked I could find copies in the $400 range. Any idea where there's one for a reasonable price?

Gasoline engines don't need spark ignition to burn, but the uncontrolled flame front propagation will kill the engine in no time.

Honda experimented with "Radical Burn" technology

Activated Radical combustion. It was a way to make cleaner burning two stroke engines. Basically it worked by having an exhaust port valve that could shut down more of the port than is usual. Due to incomplete scavenging there would be some left over fuel molecules still burning (the activated radicals) when the fresh fuel/air mix came in, and would ignite it. This can happen on regular two strokes (usually air cooled) when they get too hot. They make more power and the kill switch does not kill the engine. Which is alarming when it happens to you! It has more efficient combustion than regular spark ignited combustion.

Other had tried before but Honda found a way to control it. They had experimental bikes they ran in Paris-Dakar and Baja where they did well (5th in P-D, 1st in Baja). They also made a production version, a 250cc dual sport bike, which they sold in many markets but not the US to my great disappointment at the time. I had a chance to ride the Baja bike and it was the smoothest big bore two stroke I'd ever ridden, with a very linear power band but still had good power up top. Kind of what you'd expect from a big budget factory effort, but made for long races where rider fatigue is a factor.

 

[90cummins]

Unless you have ideas about improving the design it isn't important. What is important is keeping engines in their power band as much as possible. One reason diesel engines have a reputation for longevity is because a high percentage has traditionally been operated by professionals and operators with training. As witnessed by the number of posts here about overheating, armatures are rapidly changing statistics.

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

Gasoline and diesels are totally opposite in how they react to lean & rich a/f ratios.
Gasoline engines are sensitive to lean conditions and can overheat and and have serious consequences if operated lean for prolonged periods. At rich a/f ratios gasoline engines run cooler. Diesels however thrive running at lean a/f ratios and are very fuel efficient in these conditions. However when diesels run excessively rich (rolling coal) due to over fueling or restricted air intake (clogged air filter) the EGT’s can climb rapidly melting pistons & other expensive stuff.
90cummins