Alternative Compression Ignition Fuels

[Long_Range]

This may seam dumb questions to some. However I've asked for years and don't seam to get answers from the general public.


Question number one:


Are the large propane / natural gas engines spark or compression engines?
I'm referring to the 7+ ton engines I used to see in use as stationary power.
They had huge fuel injector pumps. I never looked for a spark plug.



Question number two:


Do compression ignition engines running on gasoline become inefficient?


Regarding multi fuel compression ignition engines.
As a youngster in the early seventies I encountered the Continental diesels in the US Army “Deuce and a Half” trucks. They had a selector switch to set to an alternate fuel source. Right DOWN to gas. I was never an operator of those trucks or a member of the US Army. So I never read the manual. These engines may have actually had spark plugs to enable gas mode? I'm clueless. They were in US military trucks for decades.


Question number three:


Is a multi fuel compression ignition engine something that is economically feasible?


Question number four:


Does anyone here knows how well those old Continentals ran on gas?
I'm thinking not to well.

 

[supton]

Hmm, what little reading I've done on the multi-fuel piston engines in the deuce-and-a-halfs is that they could go up to 84 octane gas--but no higher. I guess they needed to cut it a bit with diesel if they were going to fill up mostly with gas. So, I think those trucks used a conventional diesel setup, but did not use the fuel to lubricate anything. Other wear surfaces (stuff in the injectors) are probably some high grade metal, and probably expected to be changed out far more often than you or I would think of for doing on our cars--it was a military application afterall. Given by the smoke I've seen come out of them, I also suspect they ran really low injector pressures too.

 

[MrMopar]

Question number one:

Are the large propane / natural gas engines spark or compression engines?
I'm referring to the 7+ ton engines I used to see in use as stationary power.
They had huge fuel injector pumps. I never looked for a spark plug.

Probably spark ignition. Propane has a very high octane rating. Octane is the opposite of cetane (what you find in diesel fuel). The higher octane a fuel is, the more it resists ignition through compression. IIRC, propane has an octane number somewhere above 100. I believe it completely resists ignition in diesel engines to a point, and propane injection is only helpful when there is a diesel fuel injection that acts as the ignition source for the propane.

Question number two:

Do compression ignition engines running on gasoline become inefficient?

Yes, by virtue of gasoline having less energy content that diesel fuel. A compression ignition engine using gasoline would still be more efficient than a regular gasoline due to lack of a throttle plate that creates energy loss through pumping losses. Gasoline is not at all compatible with a diesel engine because gasoline is a fuel specifically designed to resist compression ignition. To ensure reliable ignition, you'd have to up the compression ratio of a diesel engine to some figure far above modern diesel engines. Even then, gasoline is a fuel that basically "explodes" when it is ignited - compared to a much more slow and even "burn" of diesel fuel when injected into a compression ignition cylinder. Reliability begins to be a problem when you have to have an engine with compression high enough to use gasoline as a fuel.

Compression ratios of engines have a "break even" point where you can gain efficiency by increasing compression up to that point, but above that point you begin to loose efficiency because it takes more energy for the engine rotating assembly to compress the air and fuel by the piston that is on the compression stroke. You begin to rob power from the piston that is on the power stroke, to the point where you begin to take more energy away from the engine just to keep it running - and that is stealing energy from the crankshaft that could otherwise be used to power the vehicle. I believe optimal compression ratios are about 16:1 or 17:1. Below that (gasoline engines, about 10:1 ratios) efficiency could be gained by increasing compression, but then you'd have to have gasoline with very high octane ratings to ensure that pre-ignition won't be a problem. Direct injection gasoline engines might solve this problem, and you can see VW and Audi non-turbo direct injection engines have compression ratios of about 12:1. That's much higher than a typical 10:1 ratio in other cars. The turbo direct injection (gasoline) engines are showing compression ratios of about 9.5:1. Older turbo gasoline engines used to have about an 8:1 ratio to avoid knocking from the turbo boost. Older diesel engines used to have compression ratios of 21:1 or 22:1. Now that exhaust aftertreatment is advancing, and other tweaks are being made to the injection (such as finer spray patterns to cut down on emissions, and multiple injection events per combustion cycle) newer diesel engines from VW and BMW are having compression ratios moved down to 19:1, or 17:1. I think BMW might even have it as low as 16:1.

Regarding multi fuel compression ignition engines.

I don't know how the Continental engines in a deuce+1/2 work. A somewhat educated guess would be that they have sky high compression ratios, and the fuel selector switch does a few things to adjust injection timing to fine tune ignition. With zero emissions regulations for the military to follow, they don't have to worry about what comes out the tailpipe.

Question number three:

Is a multi fuel compression ignition engine something that is economically feasible?

Probably not at all. Engines have to be tailored to emissions regulations - and that pretty much means being restricted to one fuel type. Any multi-fuel engine would either be excessively expensive to meet emissions, excessively complicated to meet emissions, or probably both.

 

[GoFaster]

Q1 - A1: There are two ways to use gaseous fuels in a large piston engine of that sort. Both have some degree of real world application. Method 1 is to throttle the intake air, feed the engine stoichiometric air/fuel ratio, and use spark ignition exactly the same as for a normal gasoline engine (although the gaseous fuels have very high octane ratings and can therefore survive much higher than normal gasoline-engine compression ratios). Method 2 is to retain the diesel injection system and feed the engine a lean mixture of gaseous fuel and air. You regulate the quantity of the gaseous fuel for load control, but the air remains unthrottled. The diesel injection system stays at minimum fuel delivery. The little squirt of diesel acts like an ignition source to light off the rest of the air/fuel charge.

Q2 - A2, and Q3 - A3: Gasoline isn't suitable for a diesel engine that has been optimized for the diesel combustion process. (Feed a TDI engine gasoline, and it won't run.) But, special combustion chamber designs are possible that allow multi-fuel operation. Have a look around for the M-A-N combustion system - that was another way to do it - I don't know how the army truck engines dealt with it. Basically, in the M-A-N system, the piston had a hemispherical bowl in it (unlike the "Mexican hat" re-entrant bowl on a modern diesel) and swirl-generating intake ports (much like we have). The fuel delivery was at a relatively low pressure (compared to nowadays) and it was aimed deliberately at the wall of the bowl. Initial start-up was with glow plugs. When the engine got going, the piston walls got good and hot, and this served to vaporize the reluctant heavy fuels, and the stratification seemed to somehow reduce the ignition-delay effect of lighter fuels. Modern diesel combustion systems all focus on *not* letting the fuel contact the chamber walls. I doubt if the M-A-N system will match efficiency or emission performance of modern diesels. Its relative insensitivity to fuel quality was the big advantage.

By the way, if you go to www.elsbett.com and poke around with the engine that they developed specifically for vegetable-oil fuels, it has remarkable similarities to the M-A-N combustion system.

It would be an interesting exercise to construct that combustion bowl from a modern ceramic material so that it could run wayyyy hotter, and then see what happens ...

Another multi-fuel design was the Ford PROCO system from the late seventies and early eighties. It was somewhat like what was described above, but used a spark ignition source in the path of the injection spray. I don't know whatever came of this research.

 

[Long_Range]

Thanks for the response guys.
As always this forum is a fountain of information.

 

[Pat Dolan]

Actually, there is another way to use mulit-fuels in a "sort-of" CI engine.

You can use catalytic ignitors/plasma ignition (similar to the glow plugs on a model aircraft engine, only much more sophisticated) and aspirate almost ANY fuel. I have driven a 400 Ford on 2/3 ethanol and 1/3 water, but the same engine can be run on diesel. The leading researcher (Mark Cherry @ www.smartplugs.com) has even run a demo by flying an aircraft to a major even on avagas, draining the tanks, removing the mags and harness and screwing catalytic ignitors into the plug holes, re-filling with Jet A and flying away (with considerably better performance).

Gassers can be converted to do this easily, but when using appropriate fuels, the advantage of the diesel or dedicated engine high CR is lost. Of course, the easy (heh, heh) answer is to boost the daylights out of it to get the optimum cylinder pressures!

 

[Long_Range]

That was interesting. No mention of how much they may cost.

What sparked my interest into multi fuel diesels was generator sets.
While looking around I noticed you can buy spark ignition engines to run with gasoline, LP gas and natural gas. Just flip a switch to change over. Looked like a great idea to me. Might come in handy after a natural disaster.

Those Smartplugs Might be just the thing for a genset. However if you used them on a 5hp Honda engine would the plug cost more than the engine?

I did google up a multi fuel two stroke diesel. At http://www.2si.com/
It's a low compression light weight spark ignition engine. Odd design.
But it will run on gasoline, JP5,JP8,DF1 or DF2.

I read where down in Brazil they are starting to multi fuel vehicles with CNG (compressed natural gas).

 

[Pat Dolan]

smartplugs are purely for researchers, at this point in time, but are very simple and cost a few hundred bucks to make (each is custom built). However, they can handle a very wide range of fuels - sometimes with the same geometry (which controls timing).

 

[TDIMeister]

Question number one:

Are the large propane / natural gas engines spark or compression engines?
I'm referring to the 7+ ton engines I used to see in use as stationary power.
They had huge fuel injector pumps. I never looked for a spark plug.

Probably spark ignition.

To only add to what have already been stated by MrMopar and GoFaster, most stationary gas engines are of the spark-ignition variety. Cummins and Westport Innovations have an exclusive joint-venture technology that combines a Cummins compression-ignition engine in the B5.9-class and larger with special fuel-injection equipment from Westport that allows the engine to operate in compression-ignition mode fuelled with natural gas (that would ordinarily not self-ignite due to the aforementioned high octane rating), using just a small quantity of Diesel to act as a pilot ignitor to get the burn going.

Question number two:

Do compression ignition engines running on gasoline become inefficient?

Yes, by virtue of gasoline having less energy content that diesel fuel.

Absolutely not. Efficiency is calculated on the basis of the energy coming out as work from the energy latent in the fuel. A fuel that has less energy content does not necessarily result in a lower efficiency. Other factors may dominate that may be decisive for the efficiency of the engine, such as flame velocity, heat of vapourization, etc., but by definition less energy content is NOT at all by itself the basis for less efficiency developed by the engine. For example, a spark-ignition engine with not other modifications can achieve a higher brake thermal efficiency running on methanol or ethanol compared to regular gasoline, although the heating values of both alcohols are far below that of gasoline. Pay attention: this does not mean you will get better fuel consumption on a gallon-to-gallon basis! Why? Both methanol and ethanol have higher flame velocities, which means combustion can end faster, and therefore the cycle operating closer to a constant-volume one, which gives the maximum efficiency for a given compression ratio in comparison to a constant-pressure cycle.

Even then, gasoline is a fuel that basically "explodes" when it is ignited - compared to a much more slow and even "burn" of diesel fuel when injected into a compression ignition cylinder. Reliability begins to be a problem when you have to have an engine with compression high enough to use gasoline as a fuel.

This is an oft-misunderstood term, starting with the word "explosion," which should be banned from the lexicon whenever talking about any kind of engine. Gasoline does not "explode" and Diesel just "smoothly burn." If we want to talk about flame velocity, both fuels are roughly equal given the same air-fuel ratios. One does not "burn" appreciably faster than the other. Methane and Methanol have both Diesel fuel and gasoline beat for burning velocity, and hydrogen tops them all by wide margin.

That gasoline and Diesel fuel combust in such different manners in their respective engines are due to the states at the start of combustion and the different burning processes themselves, not any particular inherent properties of the fuels. Yes, any combination of flashpoint, autoignition temperature, volatility, octane-/cetane ratings all will play a role in the propensity for a fuel to burn spontaneously. If anything, Diesel fuel is more "explosive" in that vapours just have to sit in nothing more than an environment of about 210 degrees Celsius before it will burn spontaneously all by itself, without even the need for a spark, while gasoline will need about 50 degrees Celsius more.

Compression ratios of engines have a "break even" point where you can gain efficiency by increasing compression up to that point, but above that point you begin to loose efficiency because it takes more energy for the engine rotating assembly to compress the air and fuel by the piston that is on the compression stroke.[/quote]
Efficiency gains diminish because frictional and heat losses reach and eventually overcome any further gains that can be attainable with increasing compression ratios.

Question number three:

Is a multi fuel compression ignition engine something that is economically feasible?

Yes, depending on application involved. See above and search Cummins Westport.

 

[david_594]

Compression ratios of engines have a "break even" point where you can gain efficiency by increasing compression up to that point, but above that point you begin to loose efficiency because it takes more energy for the engine rotating assembly to compress the air and fuel by the piston that is on the compression stroke. Efficiency gains diminish because frictional and heat losses reach and eventually overcome any further gains that can be attainable with increasing compression ratios.

Arnt the pumping losses of high compression engines also heavily impacted by RPM's? Such that compressoin can be raised more on lower speed motors suffering less ill effects. A diesel with a 4000 rpm redline will be hurt much less than a motorcycle with a 14000 rpm readline?

 

[GoFaster]

Pumping losses <> compression ratio; nothing to do with each other. The losses with too-high compression ratio are from heat losses due to higher temperatures involved, friction due to higher forces involved, and a number of other factors.

The engine size relationship to compression ratio is not simple, either. Comparing a large diesel to a small motorcycle gasoline engine is completely unfair. Let's stay gasoline to gasoline, and diesel to diesel.

A gasoline engine that revs high and has a relatively small cylinder bore (e.g. small motorcycle engine) will be less prone to detonation than one that revs low and has a relatively large cylinder bore. In simple terms, the small high-revving engine burns all the mixture in the cylinder normally before it has a chance to self-ignite. Combustion takes longer in a cylinder of larger size, and it takes longer in an engine that turns slower (mostly due to proportionally less turbulence in the intake charge), and that gives more chance for self ignition. Consequently, for equivalent octane rating of the fuel, a small displacement (per cylinder) engine tends to tolerate higher compression ratios than a large displacement (per cylinder) engine. This in no way implies that the small engine will be more efficient, because there are a ton of other factors involved. For one thing, the small-cylinder engine tends to have more surface area (for heat loss) per unit of displacement.

In day-to-day riding, my 150+ hp Kawasaki ZX10R (998cc 4-cyl 16-valve liquid-cooled) has the same fuel consumption as my 55 hp Yamaha FZR400 (399cc 4-cyl 16-valve liquid-cooled). Granted, the Kawasaki is fuel-injected and benefits from being an 18 year newer design, and the Kawasaki wants premium fuel, while the Yamaha works fine on 87 octane.