Please explain this difference between diesel and gasoline engines

[zzvyb6]

This is non sense!

He's correct. Diesel fuel is added AFTER the air is preheated by compression. In a spark ignition engine, the fuel mixture is BURNED with a flame front advancing from the spark location.
IF the mixture happens to ignite by compression before the spark starts the burn, it's called 'knock'. And if this occurs before the piston reaches TCD, the extra load trying to reverse the piston early can deform bearings and crack pistons or damage head gaskets. I've even seen a connecting rod fail as if the engine had hydro locked.

 

[DarkBlack]

He's correct. Diesel fuel is added AFTER the air is preheated by compression. In a spark ignition engine, the fuel mixture is BURNED with a flame front advancing from the spark location.
IF the mixture happens to ignite by compression before the spark starts the burn, it's called 'knock'. And if this occurs before the piston reaches TCD, the extra load trying to reverse the piston early can deform bearings and crack pistons or damage head gaskets. I've even seen a connecting rod fail as if the engine had hydro locked.

You’re kind of right, kind of wrong.
Spark ignition gasoline engines actually spark, and start the burning process, well before TDC, while the piston is moving towards the cylinder head, similar to a diesel.
Knock is an untimed, uncontrolled accelerated burn, before the spark event.
Both knock, and normal spark events happen before TDC

 

[dfkrug]

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

My old John Deere 955s (Yanmar 3-cyl port injected) were only 18:1 compression ratio. If I recall, all of my 3-cylinder Kubotas were DI and in the low 20s CR.

 

[DarkBlack]

My old John Deere 955s (Yanmar 3-cyl port injected) were only 18:1 compression ratio. If I recall, all of my 3-cylinder Kubotas were DI and in the low 20s CR.

 

[zzvyb6]

You’re kind of right, kind of wrong.
Spark ignition gasoline engines actually spark, and start the burning process, well before TDC, while the piston is moving towards the cylinder head, similar to a diesel.
Knock is an untimed, uncontrolled accelerated burn, before the spark event.
Both knock, and normal spark events happen before TDC

The peak pressure buildup from the burn is timed to produce the 'ideal' pulse timed with the change in piston velocity. I've seen hundreds of high speed videos and pressure /velocity measurements while at the GM Emissions lab in Milford, MI. It's obviously a function of crank rpm, too. Nowdays, electronic controls also participate in the combustion process because emissions generate by an 'ideal' burn (mainly nitrogen oxides) are counter to EPA laws (especial the California versions). That's what we call 'Non-stoichiometric' combustion. Besides, excess fuel and downstream air injection are also needed to heat the catalytic converter to oxidize carbon monoxide. One of several reason your vehicle doesn't get optimum gas mileage.

 

[dfkrug]

View attachment 1942973

Oops. You are right. I got them backwards. I was thinking of the intake heating element used instead of glow plugs. Sometimes, memory fails. Good catch.

 

[Riding a Relic]

He's correct. Diesel fuel is added AFTER the air is preheated by compression. In a spark ignition engine, the fuel mixture is BURNED with a flame front advancing from the spark location.
IF the mixture happens to ignite by compression before the spark starts the burn, it's called 'knock'. And if this occurs before the piston reaches TCD, the extra load trying to reverse the piston early can deform bearings and crack pistons or damage head gaskets. I've even seen a connecting rod fail as if the engine had hydro locked.

Diesels inject fuel BTDC. Depends on the engine to what degrees you set it on. 18 BTDC was common if I recall correctly. So it's close up there, but not at 0. The amount of fuel is controlled in the old injection pumps, and is controlled by HEUI systems now. Common rail fuels systems now are controlled by the PCM with stacked capacitor injectors. There are even more variants out there. This is just off the top of my head. The properties of the fuel change the whole game, as far as comparison. I have run diesels on used cooking oil my class brought into the shop when I was an instructor. They wanted to know the next day why the whole building smelled like someone frying chicken. I told the car people why, and their mouths dropped open. They always knew the next day I had been running an engine, of course I jacked with the timing so to really give them something to complain about.

 

[Riding a Relic]

Diesels are only "lean" if you think about them like gas engines where the air and fuel are completely mixed. They're not. Think of a diesel idling. It's still getting a full flow of air (no throttle plate) but only tiny squirts of fuel. Only the area right around the injector will have fuel concentrated enough to burn. The rest of the chamber's mostly air, with maybe a small amount of unburnt fuel. When the fuel droplets are too far apart there's no chain reaction to maintain combustion.

I dont think EFI vs MFI makes a difference in the basic theory of how the two systems differ in combustion. Same with IDI and CVCC.

Modern EFI gas engines can run pretty lean compared to the old days but they're nothing like diesels. And they can only do it with knock sensors and the ability to fine tune the fuel and timing.

Actually, the injector atomizes the fuel. So the idea that just around the injector will have a flame front is not entirely correct. I have a tool that I can actually see the flame in the cylinder. It's always white hot at idle and throughout the combustion event.

 

[Riding a Relic]

You’re kind of right, kind of wrong.
Spark ignition gasoline engines actually spark, and start the burning process, well before TDC, while the piston is moving towards the cylinder head, similar to a diesel.
Knock is an untimed, uncontrolled accelerated burn, before the spark event.
Both knock, and normal spark events happen before TDC

Why are the newer diesel engines a lot quieter than they used to be? Because some electronic engines atomize the fuel in as many as 6 times before the final event where they make the power. With different amounts of fuel. Comparing diesels to gasoline engines is like comparing apples to oranges, except intake compression power exhaust. Unless it is of course a two stroke diesel engine. Or, an opposed cylinder diesel engine. As in the Napier Deltic engines.

 

[Grumpycat]

Why are the newer diesel engines a lot quieter than they used to be? Because some electronic engines atomize the fuel in as many as 6 times before the final event where they make the power.

Because a judicious mix of exhaust has been added along with compression from the turbo charger to create a slower combustion, to control the combustion byproducts. Also combustion is timed by the Direct Injection.

Cant "atomize the fuel 6 times" from a direct injector.