Please explain this difference between diesel and gasoline engines

[Thereisagod]

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.

12.5:1 to 14.7:1 compression ratio in a petrol motor? I believe you're talking high performance sports cars and certain sports motorcycles. Most petrol engines are around the 10:1:1 to 12.5:1 ratio, even lower - unless I'm way out of date

 

[charlessenf]

It's actual experiences from people other than you.

Yes, that is called anecdotal information. Collect enough, verify details, add missing details, organize it logically and you've a potentially valuable database.

One thing about us humans is that we are generally unaware of what we are unaware of, tend to operate within the parameters of our individual experiential parameters (comfort zone).

Years ago when they first came out with the little tubes and egr valves and such to reduce emissions from my sixty-something Buick, I had a mechanic 'fix' an issue by cutting one of those 'extra' tubes and plugging the ends with a couple of bolts.

Now, some engineer working for GM with a fancy ass laboratory and all manner of air quality testing apparatus had designed thatl arge Buick engine and al the parts on it to function properly - including that black rubber tube.

That 'mechanic' has a story to swear by about how he 'fixed that Buick' he'll swear by. And, I have another.

 

[Rancher Ed]

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.

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.

 

[JRH02]

Just build a triple turbo 12v Cummins at 700+ hp and have the best of both worlds.

 

[pmsmechanic]

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.

The further north you live the more the ribbon heater is used for cold weather starting. Believe me on this.

Crank time on HPCR is longer because the engine needs to crank over enough times for the computer to figure out where tdc is. Once it knows that then it can compute the injection timing.

 

[Bentrim]

Yeah I know another long winded post--- Just hope it is worth the read. I know some can add to or even correct this information, but use it for what it is worth to you. Enjoy

The difference is in the properties of the fuel. Both most be atomized to burn properly.
Gasoline is usually mixed with air before it enters the cylinder and is compressed as a mixture to be ignited by a ignition source. That said I understand now there are direct injection gasoline engine, and I have not studied them yet.

A gasoline engine always has the air throttled and therefore is controlled on how much air/fuel mixture is allowed to enter the intake manifold and the cylinder.
Early farm tractors running "tractor fuel" (kerosene or distillates) had compression ratios down around 5 to 1, as gasoline became cheaper and more available compression ratios rose to 7 or 8 to 1.usually I have not found one above 8 to 1. Many of these low compression tractor could be "updated" to burn gasoline by replacing the pistons and intake/exhaust manifold. And then under extreme load preignition could be a problem. Even at these low ratios some attempted to use water injection to control preignition. Back in the muscle car era the higher compression of 12 1/2 to 1 was about the highest that could be safely run with Sunoco 260 the highest octane available on the street.

Gasoline start Diesel engines.
Early development of the Diesel engines found current electric starters did not have enough oomph to start them. Caterpillar used a hand started gasoline engine to start the Diesel. John Deere used a two cylinder engine on their first Diesel model R, For the next model 80 and later 2 cylinder Diesels they used a gasoline V4 until electrics caught up in about 1958, even then a compression release was still needed. IH on the MD used a 4 cylinder Gasoline/Diesel engine. (One side gas, the other Diesel)Pulling a lever opened a chamber in the head that lowered compression and exposed spark plugs. The engine then could be started on gasoline, after it warmed up the lever was pushed front and the engine was running on Diesel.

Diesel
Most Diesel engines run on an unrestricted air supply, Yes I have seen some with a throttle plate in tha air intake manifold (BMC Diesel in a Nuffield tractor)
Early Diesels used a energy cell or often referred to as a precombustion chamber. This help in many ways as it was easier on parts, and had less "Diesel knock", made it easier to build current production gas engine with a few modification to Diesel. Most early designs used an injector on one side of the head and the precombustion chamber on the opposite side, so the injection "shot" the fuel over the piston and into the cell (Oliver and MM), while some injected fuel directly into the cell (IH). The disadvantage was they were usually harder starting and required a starting aid when cold. Even some needed glow plugs after just being shut off!! (IH 282 in 560 and 706)
Direct injection eliminated a lot of issues, was more efficient, easier to cold start, but rattled more. They were also built heavier. The injector shot the fuel into a combustion chamber made in the piston. Even then some were very quiet as for Diesel noise (MM 585 in G1355 and White 2-150)

Diesel Fuel Injection
Not familiar with early fuel injection or new electronic fuel injection
Most early farm tractors used a rotary or inline injection pump. The rotary (CAV, Roosa Master, some Bosch) used a cam the "pushed" rollers together to generate the pressure required to pop the injectors, some had a timing advance built in and some did not advance.
Inline pumps used a row of piston/cylinder assemblies the ran off a camshaft to generate the high pressure.
All were governor controlled and varied the amount of fuel injected.
Cummins using the PT systems used the engine camshaft to operate the injectors and the amount of injection was controlled by the pressure in the fuel manifold.
I know Detroit was similar and used the camshaft but am not familiar with the control.
Modern electronics can vary timing, amount of fuel and even have multiple injections. Great for all around performance until the computer dies. That explains why they a so quiet and can produce so much power while reducing pollution.

 

[Willie B]

Diesel fuel is about 1.5 times the energy potential in a given volume of fuel. Diesel explodes more steadily expanding to push the piston down, less of a bang that gasoline gives. More power is extracted from equal energy from a diesel. Because compression is greater, motor components must be stronger.

 

[Brendanspapa]

In a diesel engine, the fuel burns relatively slowly, as it's injected into the cylinder, rather than rapidly, as in a spark ignition (gasoline engine). Further, the combustion (burning) process progresses only as it encounters enough oxygen in the cylinder, which may be further away from the point of injection. Combustion occurs at a constant pressure in a diesel, vs at a constant volume, in a gasoline engine.

 

[CalG]

As so many have offered, the difference is the fuel!

It's like comparing TNT with a lump of coal.

Both release heat on combination with reactive chemistry elements. But under controlled conditions, at different rates. Rate of heat generation results in Temperature.

Powdered coal can be very explosive... so can wheat flour.

Producer gas is a viable fuel source in desperate conditions.

 

[topgun05]

Crank time on HPCR is longer because the engine needs to crank over enough times for the computer to figure out where tdc is. Once it knows that then it can compute the injection timing.

Maybe in the most simplistic design IF motor stopped in worst spot it would need 2 crank revolutions for ECU to "figure it out" but anything built in this century will "know" the position in 1/4 to 1/2 of revolution no matter where it stopped.