Please explain this difference between diesel and gasoline engines

[Riding a Relic]

...and just to really mess with you, there are now engines that feature both compression-ignition and spark-ignition. Notably the Mazda Skyactiv-X technology, which has gasoline compression ratios as high as 16, instead of the 13-14 ratios in their sparky gas engines (Skyactiv-G).

Plus, Mazda's diesels (Skyactiv-D) have been running CRs as low as 14, utilizing multi-nozzle DI piezo injectors, and computer-controlled exhaust valve lift.

International worked on that a while back, the valves being operated by solenoids. They put some experimental units out there, but I don't know any more than that.

 

[Riding a Relic]

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.

It wasn't just to atomize the fuel. It was done to meet EPA regulations without a DEF system on trucks. The is no judicious mix for diesel exhaust dumped back into the combustion chamber. In my humble opinion. This, is only following the path of the automotive world on heavy trucks and equipment. They had ultra clean burning engines right after they developed electronic controls over fuel injection. I do know egr works on gas burners and I know how it dumps inert gas back from exhaust...until they came up with variable cam timing...now EGR is disappearing in the gasoline world...I guess the EPA will come up with some different regulations in the future to run engines on pixie dust.

 

[Riding a Relic]

Actually, they are closer than you think, now that Direct Injection is Widely Used in Spark Ignition Gasoline Engines.

You are correct about Multiple Injection Cycles though making the Diesel Compression Ignition Engine quieter.

So main difference is Spark vs Compression Ignition, and the Build Strength to Handle either, and Potential Energy Found in either Fuel, I believe Diesel has 25% potential energy, weather the Engine design can harness the extra or not is another topic.

Diesel fuel produces more torque, down the long cylinder, than gasoline does, simply because it burns slower. In the tractor world, torque is king. For day to day operations, diesel is there for those that have to work for a living. Gasoline used to run in tractors on the farms 50+ years ago, cause they cost less and were easy to work on for the small farmers. Now, the small farmers had to adapt to keep production up with a diesel tractor.
There are no simple solutions, but there are intelligent choices.

 

[D'oh]

Yes, I own two vehicles that have direct cylinder fuel injection.

Yes, I very well know about direct injection on the gasoline engines. I know that the valves begin to neck with carbon deposits and can cause premature problems as compared to other systems for automotive fuel injections. In fact, two of my personal vehicles run with direct injection. But, they sure do have plenty of power on the open road.

[Yes, Cloaking is and issue from Exhaust Gas Recircalation, but it isn't limited to Gasoline Engines. Diesels also Recirculate Exhaust Gasses.

However, Newer DI Gas Engines have combated this with Dual Injection, washing the Intake Port. Diesels have yet to accomplish this. But most of the Diesel EGR gets clogged in the EGR Cooler before the Intake Valve.

Diesel fuel produces more torque, down the long cylinder, than gasoline does, simply because it burns slower. In the tractor world, torque is king.

Diesel does burn slower, but that isn't all that creates Torque, It's Energy Potenial is there long before Ignition.

Chemical reactions (oxidation)

Material	Specific energy (MJ/kg)	Energy density (MJ/L)
Diesel fuel	45.6	38.6
100LL Avgas	44.0	31.59
Jet fuel (e.g. kerosene)	43	35
Gasohol E10 (10% ethanol 90% gasoline by volume)	43.54	33.18

But the Internal Combustion Engine regardless of Fuel Burnt/Used, is still very Inefficient, 20-40%, the rest is unused Heat, Friction, Reciprocating movement, and so on.

Direct Injection (Timing and Atomization) helps in both cases/fuel, but at a price

 

[RayIN]

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

A diesel engine was originally named a heat engine, for obvious reasons.
A diesel at low idle uses practically no fuel, incoming air is compressed by the turbocharger, air compression keeps it turning for the most part.

For your reading pleasure: Diesel Warm Up & Cool Down Considerations

 

[RayIN]

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.

Todays diesel engines have a pre-ignition injection just prior to the main fuel injection to increase fuel combustion/burn percentage.

 

[DarkBlack]

A diesel at low idle uses practically no fuel, incoming air is compressed by the turbocharger, air compression keeps it turning for the most part.

Not really. There’s no free lunch air compression energy. The only energy is from burning the diesel fuel, and diesels are inefficient at idle, because, unlike a gasoline engine that throttles air intake thus minimizing thermal and internal pumping losses, the diesel keeps pumping in full unrestricted volumes of cold air, and pumping it out the exhaust system. That’s why diesel’s don’t warm up well when started and idled. A high percentage of heat energy is wasted heating the unrestricted air. It’s essentially an air pump at idle

 

[DarkBlack]

...and just to really mess with you, there are now engines that feature both compression-ignition and spark-ignition. Notably the Mazda Skyactiv-X technology, which has gasoline compression ratios as high as 16, instead of the 13-14 ratios in their sparky gas engines (Skyactiv-G).

It still requires an ignition system, spark plugs and a throttle system.
So it’s still a “sparky gas engine”.
To me it’s more Mazda hype than anything truly revolutionary for widespread adoption.
Plenty of other manufacturers have gasoline engine designs taking advantage of multiple DI events to create incredible torque numbers.
Take for example, Honda’s little gasoline 1.5l turbo engine they’ve been putting in CRV’s and such for years. It’s the exact same size (1.5l) as my 40Hp turbo diesel tractor, yet the gasoline Honda engine puts out more torque, and over 4X the horsepower

 

[DarkBlack]

Yes, I very well know about direct injection on the gasoline engines. I know that the valves begin to neck with carbon deposits and can cause premature problems as compared to other systems for automotive fuel injections.

Just adding to: Deposits on the back sides of intake valves, is the result of PCV system operation, not direct injection. Relying on adequate detergents in the gasoline to wash the intake valves allowed manufacturers to get complacent with PCV designs. You’ll notice PCV system designs have become much more advanced now, since they can’t rely on atomized gasoline air flow to do the cleanup up work of the mess left by PCV oils. Some manufacturers are also reportedly employing intermittent fuel injection events during intake flow reversion events to get some detergent on the backsides.

 

[Grumpycat]

Just adding to: Deposits on the back sides of intake valves, is the result of PCV system operation, not direct injection.

No, it is not. The deposits are from exhaust which exit the intake valve as it initially opens.

Relying on adequate detergents in the gasoline to wash the intake valves allowed manufacturers to get complacent with PCV designs.

So now those detergents are placed in the motor oil and rely on the PCV system to deliver to the intake valve.