Please explain this difference between diesel and gasoline engines

[etpm]

I'd like it if someone could answer the original question which in a nutshell is this: Why is a lean mixture catastrophic for a gas engine but not for a diesel? So far the majority of the discussion has not been about this.

I'm going to venture a guess that it has to do with the differences in burning characteristics between gas and diesel. But I have no idea if I'm right or not. It's just a hunch at this point.

I think you are correct in that it is the burning characteristics that make the difference. And thanks for bringing up my original question.
Cheers,
Eric

 

[Grumpycat]

I'd like it if someone could answer the original question which in a nutshell is this: Why is a lean mixture catastrophic for a gas engine but not for a diesel? So far the majority of the discussion has not been about this.

I'm going to venture a guess that it has to do with the differences in burning characteristics between gas and diesel. But I have no idea if I'm right or not. It's just a hunch at this point.

Yes. The DoD procured "multifuel" trucks after WWII that had a lever to select gasoline or diesel. GIs found it ran like a scalded dog with gasoline in diesel mode... until the engine grenaded.

 

[ericm979]

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

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.

 

[Riding a Relic]

Injection timing is critical to a strong running engine. Diesels ignite the fuel due to heated air when it is compressed, this makes compression critical. A lot of diesel engines produce only 300 to 400 at a compression test (crank over, no start) but once running develop 1000 psi, some even more, depending on the particular engine. The fuel burns longer, usually down a longer power stroke than a gasoline engine.

Now, the injection system for fuel times the combustion event for optimum power based on RPMs, throttle input, or computer depending on the fuel system. Piston crown shape helps burn the fuel easily. Sometimes they rely on a prechamber, that may be in the head or built into the top of the piston.

Add a turbocharger or blower to the system, you get more air than you need, helping produce the ignition event. This produces more power. Remember, internal combustion engines are a air compressor.

Hope this help you understand a little more.

 

[AndrewInOz]

Only the area right around the injector will have fuel concentrated enough to burn. The rest of the chamber's mostly air,

Second that.
So at low fuel , the Diesel can tolerate the extra air ( that it needs to achieve enough compression ) because its mix is not homogeneous at the instant of ignition, burning wherever it can near usual propagation speed. At high fuel, the mix moving closer to ideal stoichiometric.

A lean petrol/air mix , already homogeneous, burns slower & down the stroke thru to exhaust so appears "hot" since its energy isn't as largely absorbed displacing a piston.

 

[Riding a Relic]

This is a tough crowd!

Just some statements off the cuff as pertains to I.C. engines.

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

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 and there are several "lean burn" schemes that have come and gone over the years. (as mentioned here by others)

Gasoline engines also rely on the "heat of vaporization" to cool the intake charge.

Alcohol burning engines such as JAP or ESO speedway machines run VERY HIGH compression ratios without detonation due to their FUEL, Alky, but still rely on spark ignition.
Methanol is Alky, and has a HUGE heat of vaporization value, Speedway bike cylinder heads are sometimes insulated to keep heat IN.

Popular in England , and elsewhere for a time, were TVO engines. Thermal Vaporizing Oil.
The spark ignition engines were started on gasoline, and after a suitable engine warm up, the fuel tap was switched to draw from a kerosene tank. This was popular for ag tractors due to fuel costs and availability. Many Fordson tractors built in Cork and Dagenham were so fitted.
This "flex fuel" desighn was an option for the E27n that I run, though the one I have is gasoline only. IIRC the engine compression ratio is around 6:1. ;-)
There was little difference in the engine internals between the TVO and a gasoline only engine., but the induction track had a Heat Plate that was warmed by the hot exhaust. I'm quite sure the engine would run just fine on gasoline but perhaps there would be a tendency to knock under load.

I am not aware of any Diesel cycle engines that do not make use of direct injection. The pressurized spray being required to atomize the heavy fuel being necessary I suppose.
Ocean going diesels burning "bunker crude" require fuel heaters to liquefy the fuel to make it suitable for injection.
Compression ignition engines Diesels, come in both four stroke and two stroke designs.
The "Detroit Diesel" the flag bearer of the two stokes.

I have not seen any "rotary piston" compression ignition engines . Wankels would seem to be a great platform, but not yet ;-)

I could go on, Please add in any otherr gems you might be holding back. I have engoyed the IC engine all my life, I must have 40 of them all counted .

Now don't get me started on glow ignition. I can smell the "Missile Mist" now.... ;-)

There are many diesel engines that use a pre chamber in the head firing deck. It gives them a tighter area to burn the fuel at the beginning of the firing event. Direct injection sprays fuel directly on top of the piston on it's way up and they are shaped different to allow for this. The larger the engine the less use of pre chambers you find.

 

[walnutman]

Diesel fuel is much easier to store than gasoline. It doesn't degrade nearly as quickly as gas.
Most modern diesel engines use super high injection pressures in the 30,000 psi range. HPCR (High Pressure Common Rail) This allows the injectors to highly atomize the fuel. Also the computers are able to fire the injectors multiple times in one power stroke. Ever wonder how they got diesels to be so quiet? They actually spray a shot early to start the combustion so as not to create as violent of an explosion. Then the injector fires again, often multiple times. And yes in diesels there is fuel timing. It is similar to spark timing with respect to the timing of the injection relative to piston position. Timing is controlled by computer.
Another great advantage of HPCR is cold weather starting. Much easier to start.
I have 2004 Dodge Cummins. It is HPCR. The only cold start assistance it has is a ribbon heater in the intake horn. I can start that engine in below zero conditions without waiting or plugging in. Granted it will crank a few seconds longer but it always starts.

 

[aircommuter]

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.

Musk is not an inventor just an investor.

 

[aircommuter]

Diesel fuel is much easier to store than gasoline. It doesn't degrade nearly as quickly as gas.
Most modern diesel engines use super high injection pressures in the 30,000 psi range. HPCR (High Pressure Common Rail) This allows the injectors to highly atomize the fuel. Also the computers are able to fire the injectors multiple times in one power stroke. Ever wonder how they got diesels to be so quiet? They actually spray a shot early to start the combustion so as not to create as violent of an explosion. Then the injector fires again, often multiple times. And yes in diesels there is fuel timing. It is similar to spark timing with respect to the timing of the injection relative to piston position. Timing is controlled by computer.
Another great advantage of HPCR is cold weather starting. Much easier to start.
I have 2004 Dodge Cummins. It is HPCR. The only cold start assistance it has is a ribbon heater in the intake horn. I can start that engine in below zero conditions without waiting or plugging in. Granted it will crank a few seconds longer but it always starts.

The ribbon heater isn’t for starting just for emissions reduction while cold. Crank time is longer on HPCR because the injection pump needs some speed to develop the pressure.

 

[aircommuter]

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

The simple answer is it’s the fuel characteristics.