Hello all,
I am working in the heavy truck industry now and am interested in learning about some of the differences between gasolene engines and diesel powerplants. I know the basics but am curious as to why the different approach was taken.

I knew that diesel engines did not use a spark to ignite combusiton, but was always perplexed as to how they would get the A/F ratio to be perfect eough to cause ignition at TDC, expecially because of the fact that most all diesels are turbocharged and A/F ratio would vary depending on boost. I think some recent articles about direct injection have cleared up some of this confusion. I am guessing that diesel engines are direct injection and inject fuel at top dead center directly into the combusiton chaimber.

My question is: in diesel engines how does the air and fuel properly mix before combustion? I was talking with my dad about this (started as a diesel mechanic and has worked in the heavy truck industry for 35+ years) and he thought it was because of the extremely high compression ratio in a diesel engine. He was saying that the air charge is so dense that it mixes properly.

So why haven't gasolene engines uses direct injection forever? Why not just crank up the compression, throw on a turbo and use direct injection? I am guessing it has something to do with the chemical differences between gasolene and diesel, but this is where any knowledge or intuition I have stops. Why did gasolene engines evolve to have fuel injected into the intake runner to be ignited by spark while diesel went the direct injection route without spark?

TIA for any insight

 

You are on the right track. Diesels are "compression ignition" (CI) engines. They compress the air so much it gets hot enough to burn and right at that time diesel fuel is directly injected into the hot air to start combustion. More fuel means more power.

"Spark Igniton" (SI) engines generally mix the fuel with the air before it reaches the intake valve, compress the fuel-air mix and use a spark plug to light it off. Recently "direct injection" SI engines have started to appear. The can be somewhat more fuel efficient, but they cost more to build because the injection is under very high pressure. They are not all that much more efficient, but enough to warrant thier use is some applications.

Cost is the main reason gasoline SI engines haven't used direct injection forever. Large diesels use mechanical injection which means more parts.


Check out www.howstuffworks.com for more informaton.

 

Cost, packaging, and elusive benefit all have worked against direct injection. However, there are quite a few direct injection gas engines today...from VW/Audi, MB, BMW, Toyota.

It is the computer controls and high pressure systems that now let diesels compete with spark ignition. In my opinion they are way better for non-performance applications. My '09 I expect diesels to flood the US market. My '15 I think they will be dominent. Hybrids that exist will be diesel-electric hybrids. However, the hybrid advantage is smaller with a diesel.

Until recently I don't think diesels have been focused on the quality of combustion. The diesel's advantage is lower pumping losses since the throttle is always wide open and power is controlled by the amount of fuel injected. As more was injected, they became less efficient (witness the black plumes of smoke). The computer controls minimize that.

 

It also apparently takes 22,000psi to successfully directly inject gas into the chamber - and that technology has not been available for long.

There's more energy per gallon of diesel than in gas - a surprising fact. Gas makes more power I guess because it burns faster - but as diesel technology improves - it could actually pass gas for power output per cubic inch one day... wouldn't that be interesting?

Any gas engine can be converted to run without spark. Sonex Research in MAryland leads the field here. If their technology ever takes off you will not need the heavy engines needed at present to run compression ignition because they do not need 18-22:1 compression ratios.

They can fire compression ignition engines as low as 6:1 - but between 9 and 12:1 is ideal - making the present performance gas engines perfect for conversion to compression ignition.

What Sonex does is remove the spark plug bosses and machine in a boss for a direct injector. They then have a custom piston made which has a proprietary design. Imagine a volcano cone with a pointed top, a valley around it that then rises to form a flat quench rim around the piston edge. The rim is hollow - with special chambers cast into them, and there are small holes drilled horiznontally around the rim vertical face that access those hollow chambers.

The angle of injection and cone shape are all very precise and the fuel (these motors will run on military JP5, gas, diesel, cooking oil - almost anything that will burn) is burned normally under compression, then excess is sent under pressure into the chambers where it expands and creates a secondary power stroke.

Because the fuel is precisely metered and timed - there is no pre-ignition or detonation problems. Professor Andrew Porring - CEO of Sonex and former Professor of engineering at the Naval Academy in Annapolis is the brain child of this - and he graciously gave me a guided tour and look at combustion maps/charts and engines under test... this could be big, I hope he succeeds as he's done a lot of work and this technology deserves to get out into the market.

They are presently developing a 400hp Subaru boxer engine for military use. Super light weight for a 400hp motor than can run on JP5. These engines do require VASTLY more air than normal spark engines. Wouldn't that make them more fuel efficient?

wink

I wanted to drop off a twin turbo Viper V10 for Andy to play with but we left the USA before I got the chance. Would have been a sweet project: 1000hp+ and 40mpg+ with cleaner emissions than a stocker.

 

Please tell me that last post was a joke.

 

I agree with you on Diesels will enter the US market and become dominant within the next few years (I would say less than 5).

In the states we worked with Cummins diesel engines that were used to transport loads of boats from factories to dealerships...

Overhere in Europe, Diesels are everywhere and have been for more than 20 years. The difference in fuel economy not only comes in regular driving, but overhere people who tow campers and such (which would be towed with pick-up trucks and nothing less in the "states") are pulled with their everyday Diesel vehicle (Toyota Corolla, VW Jetta, Mercedes 124 and C classes, etc...), a small gas engine would not only not be able to pull, but the fuel economy would be very bad.

Also Skoda (VW) just recently released their first (I believe it to be their first as far as I remember) Direct Injection engine, one down-side the average consumer may find unacceptable is the diesel-like noise of DI engines.

 

Great info guys...

So lets talk a little about the advantages of direct injection CI engines VS direct injection SI engines; and diesel fuel vs. gasoline.

From what I understand current CI engines do not work well in high RPM applications (where the power & performance is). I have even read that there is some diesel technology out there today that uses CI in the lower RPM range and switches over to SI as the rpms climb.

I am still a little confused as to how the air and fuel are able to mix well in a direct injection engine. I do know that the fuel pressure is much higher in conventional diesel engines than gasoline, so high it is almost spraying a vapor. I don’t think it is nearly as high as the number quoted above for a similar gasoline engine though (maybe gas has different properties, and requires more pressure?). Maybe the air and fuel do not mix well and this is why diesel engines are a little ‘dirtier’ than today’s gasoline engines (more Nox emissions). To meet the new emissions standards for ’07 the heavy truck industry has started the use of DPF (diesel particulate filter) units. These trap a good portion of these particles and burn them as a secondary operation in the exhaust system.

So lets talk a little about the new BMW 3.0L Direct Injection twin turbo motor that is coming out. If fuel is not injected until top dead center is there any chance for pre-detonation? If not, then it seems like you could turn the boost waaaayyyy up and make a ton of power not having to worry about detonation. But again I get back to: how do you control the a/f mix in the combustion chamber?

 

From How stuff works:
"The injector on a diesel engine is its most complex component and has been the subject of a great deal of experimentation -- in any particular engine it may be located in a variety of places. The injector has to be able to withstand the temperature and pressure inside the cylinder and still deliver the fuel in a fine mist. Getting the mist circulated in the cylinder so that it is evenly distributed is also a problem, so some diesel engines employ special induction valves, pre-combustion chambers or other devices to swirl the air in the combustion chamber or otherwise improve the ignition and combustion process. "

"Diesel fuel has a higher energy density than gasoline. On average, 1 gallon (3.8 L) of diesel fuel contains approximately 155x106 joules (147,000 BTU), while 1 gallon of gasoline contains 132x106 joules (125,000 BTU). This, combined with the improved efficiency of diesel engines, explains why diesel engines get better mileage than equivalent gasoline engines."

 

I believe the 1950's Mercedes SL300s were direct injection. What the high pressure allows is injection in a shorter period of time.

The big advantages of diesel are the use of lean mixtures rather than a throttle restriction. The higher energy content basically tells you that diesel fuel is less expensive than it seems. At $3/gal, diesel is $2.04/100k BTU vs $2.40/100k BTU for gas.

It you operated at WOT, the diesel advantage is not there. Even in Europe, you don't see diesel alternatives to 1.6-2.0 NA engines. Note that all current diesels, TTBOMK, are turbocharged.

 

Audi's turbo diesel race engine in the R10 is the favorite to win LeMans this June. There is some interesting info on in in some of the Brit car rags.

 

Was there something funny? Dang - I love a good joke. Must have missed it

That post was for people smart enough to recognise it's possibilities - sorry it went right over.

Sonex has been working on military projects primarily - go figure. I got a full guided tour not because of my charm and good looks. 30mpg out of a 500 c.i. 1000rwhp TT Viper is perfectly possible right now using old tech - so what's your problem? If you think that's dreaming - good for you bud - bye.

 

The joke was that I would like to meet the person who owns a 1000+hp vehicle and still has the restraint to drive mildly enough to get 40+mpg, even if it is theoretically possible. And no, your post didn't go over my head. Btw, there is a good reason why compression ignition engines will never surpass spark ignition engines in max power per displacement. The spark ignition engine has plenty of time to inject much more fuel than a compression ignition engine because of the constraint of having to inject fuel during a limited portion of the compression stroke. This is one of the reasons you see gas engines turning several times the rpm of diesels.

 

Can you further elaborate on why theres no advantage when going WOT? Im not disagreeing just curious for more information... When you mean advantage do you mean hp/tq, acceleration, fuel economy (which I assume you mean)... Over here in Europe it seems gasoline spark ignition is a fading era (at least in my eyes), I prefer gasoline for performance vehicles but just seems when people buy a new car the choice is for diesel.

 

Here is the basics

Diesel:

RPM and power controlled by fuel input, they are always run lean, and the engine always has the maxiumum amount of air being pumped through it. Up untill now, emmisions controls on diesels have been non-existant so AFR and combustion were never really focused on. Basically a GAS pedal in a diesel is just that, it injects more fuel into the engine. There is no Throttle body on a diesel because you dont need to worry about the engine running lean! A diesel almost never sees stoichiometric AFR due to particuate/smoke emmision (i.e. the plume of black smoke that comes out). Diesel fuel does not burn as nicely as gas, because it does not "vaporize" and become a gaseous mixture as easily. This is why diesel engines inject at such high pressures and really focus on the swirl/tumble inside the combustion chamber. Oh, and WHEN you inject the fuel determines when the "ignition" of the fuel starts.. kinda like a spark advance.. only its a fuel advance... (and it needs to be injected at a higher pressure than is in the combustion chamber.. or esle it wouldnt go in!!)

Gasoline:
RPM and power controlled by AIR intake (throttle body). Since Gasoline does not like to run lean (for emmisions and other reasons) it must maintain the proper AFR. Your tuning makes your fuel injectors KEEP UP with the air your allowing into the engine. On a gasoline engine the GAS pedal is more like an AIR pedal and allows you to give the engine more AIR. the computer compensates by adding the correct amount of gas.

 

How powerful are diesels without turbos? I drive a dumptruck and that thing is a TURD when the turbo isn't spooling!

 

we had a 39' motorhome, caterpillar 300hp diesel, 1200ft-lb.. driving up a hill at 80mph no problem, then BANG! 7-ply turbo hose blew, slowed down to 30mph, and that's all it'd do.. just chug chug chug... Diesel engines suck without turbos.

Does the engines' computer determine the amount of fuel compared to the amount of air coming in? Kinda like an AFR, but not so much. When the engine is making boost, it pulses the injectors at the right time to get the optimum power and efficency, but when the boost is suddenly taken away, does the engine realize that there is less air coming in, and therefore "cut" fuel volume into the engine? I've studied the injector mechanism and fuel pump on a diesel engine, but I never saw an airflow meter sort of thing. Since the "throttle" is always wide open, is there a MAF sensor? Or does it just run less efficently with more fuel dumped into the engine (controlled by the pedal, of course)

 

What a great site, good info

 

I apologize - I misunderstood your comment. I agree with you.

 

We have a 93 F-350 non turbo for pulling our travel trailor, and that thing is a turd. I'm lucky to be going 45-50 trying to get onto the highway. Has good torque, and can pull quite a bit though.

 

Banks makes a kit for that turd. A family friend of mine put a kit on their 89 7.3 ford and it ran 1000x better. Worth it in everyway they said. I basically paid for itself in a year and that was 6-7 years ago.