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Old 11-27-2009, 08:43 PM   #1 (permalink)
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Question Help needed from propane experts! Custom V8 Propane Turbo Performance Setup

Hi All,

I posted in the intro thread before this, so this is my first real post.

I stumbled on this forum by googling around about propane injection, which I know very little about.

I would appreciate any commentary / criticism / answers to my statements and questions below.

(EDIT: the engine I'm planning to use is a custom turbocharged GM LS3 6.2L V8)

What I do know is this: it has an octane rating of 105 - 110, and it is a gas. I want to build a really unique car, so this got me thinking - if I ran on propane, I could maintain the high compression (10.7:1) AND throw in tons of boost without running into premature detonation problems. Also, if I injected the propane pre-turbo, it would have tons of time/space and turbulance to mix very evenly with the air. I know some propane conversion companies inject supplemental propane pre-turbo in truck applications, so I figured this might be possible.

I am running a custom ECU that I designed and developed, so ANYTHING is possible from a software/hardware/sensors/etc perspective.

If I inject post-turbo, my options are limited to those that will still work under boost, so blow-through or liquid injected. From what I understand, vapour injectors are usually running at 1 - 1.5 bar, so boost would slow down (or stop) the injection rate, so that probably isn't an option, right?

If I inject pre-turbo, I also have the option of a draw-through type system or vapour injectors.

How do the systems that are designed for carburated engines handle things like enrichment during full acceleration and idling AFR changes?

I have some more questions, but that depends on what types of injection remain suitable for my application.

Any help figuring this out is greatly appreciated!

Thanks,

--Mike


Last edited by mikernet; 11-27-2009 at 08:59 PM..
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Old 11-27-2009, 08:58 PM   #2 (permalink)
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The regulators for most propane applications are set up to work with vacuum... you'd need a vacuum/boost compensator if you plan on using normally available parts, like those from forklifts and such.

Alot of information is available online, finding it can be a problem at times. If I come across any decent resources, I've got the thread open in a tab, I'll update.
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Old 11-27-2009, 09:00 PM   #3 (permalink)
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What are your thoughts on pre-turbo injection?
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Old 11-27-2009, 09:07 PM   #4 (permalink)
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I don't believe propane is detrimental to oil films, so it's probably a safer bet to inject it pre-boost, using a boost reference to calculate actual injection pressure/duration.

Vaporization isn't really an issue, as propane is gaseous at room temperature, and the likelihood of your post-boost air being anything short of 100degF is just moot.

To inject after the turbo, you'd be looking into higher injection pressures and longer injection durations, and you'd need everything under a high enough pressure as to keep the propane liquid until it was injected into the intake stream. You probably don't want to vaporize the fuel until it's been injected, due to the pressures you're likely to be working with in the intake stream.

You won't really need to run a vaporizer except on cold days, and it doesn't seem like you'll be wanting to run this thing in the winter anyway.

I think you'll probably get the best efficiency from a direct injection sort of setup, using the propane as though it were a diesel-type fuel, and varying the injection duration. It would allow you to go throttleless, and since you've got a custom ECM, you should have no problem tuning for it.
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Old 11-27-2009, 09:24 PM   #5 (permalink)
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Why would I need a boost reference at all if it is being injected pre-turbo? Wouldn't it just inject based on air flow to achieve desired AFR?

I'm also slightly concerned about the potential kaboom if there is an intake backfire with the entire pressurized intake tract filled with air/fuel, which may or may not include an intercooler (I'm also looking at liquid-to-air charge coolers).

I'm very intruiged by the throttleless idea - I've never heard of such a thing - but how would I control engine power/RPM? If I just control fuel metering then I would end up going lean during anything but WOT, which I assume would be very very bad.
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Old 11-27-2009, 10:10 PM   #6 (permalink)
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Okay, so it turns out a lot of people run propane injection pre-turbo, and if you get intake backfire, well, it usually blows off some pipes and that's that, so no biggie there (assuming things are working properly and this isn't happening all the time).

I'm still curious as to why you are saying I would need the boost reference for the mixer - it wouldn't be operating under boost, so it would be mixing just like a naturally aspirated engine.

Quote:
Originally Posted by mikernet View Post
How do the systems that are designed for carburated engines handle things like enrichment during full acceleration and idling AFR changes?
You can ignore this question...I found out the answer. For some reason I thought they were much more simplified devices that didn't have all the circuits gasoline carbs do. I think I got that idea from looking at some simple propane assist systems for trucks. But they are actually fully functioning carburators, just designed for propane.
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Old 11-27-2009, 11:31 PM   #7 (permalink)
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With Direct Injection systems, no spark is necessary, because the propane gets injected through the hole that the spark plug would normally have been in, and propane is more easily combustible than diesel fuel. It's not an easy thing to work out, though. I just threw it in there for the sake of pointing out the options. The power/RPM is modulated by controlling the injector pulse duration, thus the amount of fuel injected. The system would basically mock a diesel engine in operation. See Diesel Cycle for more clarification.

A boost reference is a "second opinion" to your feedback loop, an MAF would probably be better for a primary reference, but if you have any leaks, the MAF will not register them, where the boost reference will be able to tell the ECM that there is a boost leak (compared to the flow meter), and will be able to compensate for it.

A simple suck through carb setup would probably be sufficient for starters, but you'd need a feedback carb if you wanted ECM control of the system. This is why I recommended injection. You can more precisely control the injection event to precisely meter fuel based on the sensory inputs.
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Old 11-28-2009, 05:02 AM   #8 (permalink)
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I think it was HotRod magazine that a few years ago did a piece on a turbo'd 350 on propane that got a reliable 650 HP. I used to drive a Propane converson Chevy 1 ton on propane. Metering wasn't very scientific or highly calibrated. To start it on a cold day you simply held an electric button down for two seconds which dumped propane into the intake then hit the starter. Once it fired it ran great without hesitation. No compensation for temp or anything. Propane burns a little hotter than gasoline and is not at all as finicky with it's mixture ratio's.
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Old 11-28-2009, 07:25 AM   #9 (permalink)
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The propane conversion that one of my old bosses did was on a Dodge D50, which used a Mitsubishi engine mated to a 5 speed transmission. The Japanese 4 cylinder small truck engines are commonly found in forklifts, as well, so the parts are openly interchangeable.

His "cold start" mechanism" was just an electric heater coil in the intake, I believe. It may have also heated the vaporizer, not entirely sure. Been nearly 7 years since I worked for him, and it wasn't a long employment.
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Old 11-28-2009, 03:38 PM   #10 (permalink)
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If the MAF is right before the propane injectors, pre-turbo, a boost leak won't matter for AFR metering - the MAF is accurately reading air moving past the injectors and the boost leak is actually leaking mixed air/fuel, so it wouldn't affect AFR. A boost reference is required so that
a) you can compensate fuel pressure for the air pressure where the injectors are (which isn't an issue pre-turbo) and
b) if you are injecting AFTER the turbo, where a leak would cause a misreading on the air passing the injectors, it can be corrected

Direct injection is way outta my field - I've always disliked the way a diesel engine works for whatever reason...spontaneous combusion just seems less controllable/precise/tunable. I just don't like it lol

I think using actual injectors (be it liquid or vapour) mounted to the intake pipe right before the turbo would give me the best control via ECU. Although, injectors would have to be pulsed instead of just constantly flowing fuel into the stream of air like a carb.

Hmmm...I wonder if that would have a significant impact on how well the air and fuel mix together. Given the fact that its all going to get pushed through a turbo and 10 feet of piping, I think the difference would probably be negligable.

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