Fuel type(s) for custom built diesel engine - Educational Project

[revive]

Hello,
A team I am working with has decided to build a small, simple diesel engine as an educational project. We all have various automotive experience/training and will have the assistance of a diesel engineer, but are tasked with finding solutions on our own.. he's there to ensure the final project we spec out, is viable.. then we build it. So, the question arose... regarding fuel. From what I understand, diesel fuel uses a higher fuel/air ratio than a gas engine, is a less volatile fuel and is more difficult to atomize during injection. And we all know not to run gasoline in a diesel engine.. or can we ?? So, here is the proposed question to our team:
IF, a diesel engine is designed from the ground up - fuel injection system, etc. IS IT Possible to build a diesel engine that would run on a LESSER amount of gasoline than a typical diesel would use of diesel fuel.. running at diesel compression ratios, etc.??

It seems plausible that, if the rest of the engine system components (fuel system, injectors, etc.) are capable of providing gasoline, that the engine could in theory run off of less gasoline that it would if setup to run diesel, due to gasoline's higher octane rating and volatility.

Anyone care to chime in on this? And please remember, this is not some backyard mechanic that wants to convert his truck to gasoline!! This is for an ED project,.. and it's as much about exploring the potentials of a diesel cycle engine, as it is actually building our finalized design.

Thanks!!

 

[calteg]

Your question is poorly worded. From what I understand, it sounds like you want to build a compression ignition engine that runs on gasoline, correct?

The main issue you'll run into is pre-combustion, due to the high compression ratio most diesel motors run. The highest compression gas motor I can think of off hand is Honda's F20c from the S2000, which runs ~11.5:1 compression ratio. The Cummins B series motors run over a 16:1 compression ratio.

While gasoline tends to have a higher octane rating than diesel, this is largely because diesel has its own similar measurement called the cetane rating. Compress diesel and gasoline at the same rate, and gasoline will ignite first.

You also seem to be ignoring one of the reasons why diesel engines are efficient in the first place; diesel fuel contains more energy per litre than an equivilent amount of gasoline. Running "less gasoline that it would if setup to run diesel" also means you'll be producing much less energy.

Can you build such an engine? Sure. Lower the compression ratio(13:1?), run 100+ octane "race" gas, retard the timing quite a bit, and viola! I would suggest doing research on direct injection engines, as they share quite a few of the characteristics that you seem to be interested in.

 

[roachie]

I think he is asking if a compression ignition engine can be ran on gasoline and have a lower fuel consumption rate.

I have pondered on this. The issue I see it the poor lubrication properties of gasoline will put some immense strain on the injection system design to no seize. Another thing to consider would be the maximum compression gasoline can take before it ignites in the fuel rail or injectors.

As stated the energy contained in gas vs. diesel makes it rather pointless.

If you could make an engine run and meet emissions on bunker C it would be very interesting.

 

[6.6lEater]

I work at a GM dealer and we have something similar to what you are speaking of. Also most of the new cars comming out have high pressure gasoline injection systems injecting fuel upwards of 1600 psi with variable cam timing of +/- 40* on some models. A better idea would be to do it without a cam and make infinite timing and duration. Just something to think about.

 

[BiodieselPower]

Its been done as early as 1925. get on wikipedia and type in gasoline direct injection. many motors have already done it and more continue to go to direct injection for emmisions and economy. My guess would be in 15-20 years all engines will be direct injection...its the next step from carburation to fuel injection to direct injection fuel injection. Ski Doo even does it on two stroke snomobiles.

 

[jellydonut]

It seems plausible that, if the rest of the engine system components (fuel system, injectors, etc.) are capable of providing gasoline, that the engine could in theory run off of less gasoline that it would if setup to run diesel, due to gasoline's higher octane rating and volatility.

I think you've misunderstood octane ratings. The octane rating of a fuel tells you how resistant a fuel is to self-ignition - as you may recall, self-ignition is how a diesel engine works in the first place. The higher the octane rating, the less ideal a fuel is for the diesel process based engine.

The best fuel for a diesel engine is one with a high cetane rating - the cetane rating tells you how easily a fuel with self-ignite under high pressure and temperature. As a benchmark, take the cetane hydrocarbon itself. It has a cetane rating of 100, and an octane rating of something like 30. Such a fuel would have very low ignition lag and combust very fast, allowing you to either inject more of it per power stroke, or run the engine at higher speeds.

'Dieseling' on gasoline is going to lead to two things: ignition lag, and explosively fast combustion (knocking). In order to construct such an engine, you'd have to make it even heavier than the average diesel in order for it to withstand dieseling on a fuel with poor self-ignition characteristics. This is what they do on old multi-fuel military vehicles, and those engines don't exactly put out fascinating performance figures.

Gasoline direct injection engines like the Ford EcoBoost are somewhat similar to diesels in that they only take in air and inject the fuel once the cylinder is 'closed' (either via ports or valves), but unlike diesels they still have spark plugs, and the fuel is injected before TDC to allow air/fuel mixing, like in a conventional gasoline engine. http://www.youtube.com/watch?v=20qqavckWdw http://www.youtube.com/watch?v=YzIIoP2dz8E

The question is, why would you run a low-specific-energy, low-vapor-point, volatile, expensive fuel, when you could just run diesel instead? It wouldn't use less gasoline than diesel, because gasoline is a lighter oil and has lower specific energy. This is one of the main reasons why diesels use less fuel to begin with - the fuel itself has more oomph. Just try running a diesel on a compatible light fuel oil like kerosene, watch the fuel economy go down.

If you want to use an alternative fuel to petrodiesel, I implore you to investigate synthetic diesel.

If you are dead-set on a diesel that could run gasoline, you could investigate dual-fuel systems. Dual-fuel marine diesel engines run on both diesel and methane, a gaseous, high-octane (130 octane.. highest octane rating outside of pure hydrogen) natural gas fuel.

The engines have a dual-nozzle diesel injector mounted in traditional form in the middle of the head, and manifold methane injection similar to an Otto cycle engine with port injection. When running in diesel mode, the engine utilizes the larger nozzle in the injector and no gas is used. When running in gas mode, however, the manifold injector sprays gas into the intake air during the intake stroke. The air/methane mix is compressed during the compression stroke, and is ignited at TDC by a small spray of diesel fuel. The ignition injection is called the 'pilot injection' as it provides the pilot flame that ignites the rest of the air/gas mixture. The second, smaller nozzle in the injector is used for this pilot injection. A very small amount of diesel fuel is used for the ignition.

This is one way to do it - however, to my knowledge, this technology is currently very immature. A friend of mine has worked on a ship equipped with Wärtsilä dual-fuel diesel/methane engines, and my understanding is that the gas mode operation is only really reliable for medium loads. Low and high loads are not ideal. They also went through cylinder heads as if they were a consumable item for unknown reasons, probably the knocking issues.

Here is an excellent 3D animation showing the dual-fuel four-stroke process: http://wartsila.com/Wartsila/global/docs/en/power/media_publications/animations/DF.wmv and a different dual-fuel process with direct injection of both methane and diesel, and the diesel preceding the methane: http://wartsila.com/Wartsila/global/docs/en/power/media_publications/animations/GD.wmv

Contrasted with the methane-only, Otto process spark-ignited prechamber engine: http://wartsila.com/Wartsila/global/docs/en/power/media_publications/animations/SG.wmv

And of course the old and known diesel process, for the sake of completeness: http://wartsila.com/Wartsila/global/docs/en/power/media_publications/animations/Diesel.wmv

Here is an American company that actually sells complete bi-fuel conversion kits for various different engines: GTI Bi-Fuel Of course their target market are stationary generators so they're not exactly building kits for 5.9L Cummins engines, but it is interesting nonetheless. (or at least I think so..)