Throttleless propane, succesful test and idle discussion 2

[obanion]

Some of you may remember I have been looking into running my new motor on propane, and not using any sort of air throttle. Goal was to regulate power through fuel delivery, taking advantage of propane's excellent lean burn limits and near perfect atomization.

Before any testing I knew the biggest hurdle would be idle. The fear was that power could not be reduced enough through a super lean mixture to keep the engine at a reasonable idle speed. Beyond this, it may have turned out the mixture couldn't even be reduced enough for mild load situations (steady cruise, light acceleration).

The test I conducted this weekend put these fears to rest.

The test motor was a 3L, V6, 8.5:1 compression, motor on my wife's car (93 Dodge Shadow). Note the intended usage motor will be a 3L, I6, 10:1 compression. The motor was warmed up on gasoline. Then shut down. Fuel injectors were disconnected, and the throttle stuck wide open. A 500000BTU/hr propane torch was then inserted into the open throttle. While the engine was being cranked over, I opened the valve on the torch. After about 1 turn, the motor started without hesitation. It did not run away. Rather it stayed at about 800RPM. I then experimeted for a minute, increasing and decreasing the propane flow, and noting I had perfect control of the idle speed. More propane, motor sped up. Less, it slowed right back down. It never stalled, popped, or backfired. Finally shut off the valve and the engine instantly shut down. I do note however, that it ran a little rough, with a slight misfire.

My conclusions from this test:
Load control through fuel delivery won't be a problem. The low end of the lean limit range without misfire goes very low indeed. I have no doubts I can have mild accleration and steady cruise on all 6 cylinders without misfire or rough running. Perhaps, with the improvement from the test 8.5:1 to the 10:1 of the motor the full system is intended for, may be able to lean idle dead steady without any misfire. Also consider I had a fixed ignition advance of 15BTDC on the test motor, and will have complete control over timing on the actual motor. This could help as well. I'm not counting on that however.

I have two other options available to me to keep the motor down at a idle speed when desired.

1. Fuel cut rev limiter. I'd set my fuel map so that at 0% throttle input and 1500RPM, the fuel injected would be at the very lean limit of steady, no misfire operation. However, this will probably be slightly more power than what is needed to keep the engine purring along at 1000RPM with no external load and full atmospheric pressure in the intake. I'd use a second rev limiter (available on my fuel computer) to kick in a fuel cut only, to keep the revs down. The limiter is very adjustable, having a 0-100% level of effect in two seperate stages, as the engine revs above the targeted limit. It wouldn't be a terribly hard rev limit like you are used to thinking about either, as most likely less than 25% of injection events will need to be dropped out to keep the revs steady, rather than the 75-90% of injection events on a regular engine running full stoich or richer. Before you mention, the propane is individually port injected, which is why I can do this.

2. Run the engine on two cylinders at 0% throttle. Simply run the engine on cylinders 2&5 only (firing order 360 apart). My fuel computer can easily be made to do this as well. To maintain idle speed, I'd guess those two cylinders would probably need to run at 75-100% of stoich. Anyone care to comment if this would cause harmful vibrations, harmonics, or some other concern?

 

[franzh]

Congrats, nice work!
I would recommend one additional test. Try it with a motoring load to simulate drivetrain loads. Propane has a lean and rich combustion limits of 2.1-9.6% but can operate at regions outside of that range, but not too far. Do you have any idea of what the actual fuel use was at that load?

The misfire you indicated was the situation I discusses earlier, some cylinders will have fuel, others will not, or an insufficient amount.

When you can obtain a rock steady idle under standard loads at WOT, you will have my nod of confirmation. The rest of your discussion seems valid, assuming the above test works.

One thing to consider: At WOT, and the fuel mixture being the only speed limiting factor, ANY stray fuel anomaly will result in a run-away. Just ask any diesel mechanic who washed the air filter in solvent and relied on engine air to dry it out!
Franz

 

[obanion]

Well it's still a spark ignited engine, so any run-away will be stopped at spark rev limiter at 7000RPM, and the engine will still stop if I turn off the key (killing the ignition entirely).

I couldn't tell you the exact fuel use (didn't have a flowmeter), but it was right in line with my previous calculations. I had already determined that a 500000BTU/HR torch was perfect, allowing me to match the amount of fuel used normally during idle, with a little extra to get it higher. So I don't think I was wasting a great deal of fuel.

 

[franzh]

OOPS! Spent too much time with compression ignition engines recently!

500,000 BTU/hr works out to about 200 cu ft of raw propane per hour (2500 btu/cu/ft). Assuming your 3.0 engine was at 800 rpm, that would account for about 2500 cu ft air per hour, or about 12.5:1 air to fuel ratio by volume. This is about half of propanes ideal fuel mixture of 24:1 volume (15.5:1 AF by weight) Something doesnt make sense. Was the torch open full blast?

Please dont get me wrong, I really like your approach and applaud your efforts. This has been bantered about in lab engines for many years, myself included. I was unable to obtain transparent operation and have usable low speed engine power, although I could nurse an engine to low speeds at ridiculously high AF rations.
Franz

 

[obanion]

Sorry, was unclear.

The torch wasn't open full blast. It had a fine adjustment knob, and I'd guess it was only open at 10-20% flow at the slow 1000RPM idle.

I'm sure this wouldn't work for "transparent operation" nessasary for widespread passenger vehicle use. That's a higher standard than I require. If I can nurse the engine to low speed at the far lean limit, which I am reasonably sure I can do, that's enough. The idle can require a little bit of fuel cut (removal of 10-25% of injection events), and warble a little. No harm in that. Most likely it will just sound more "cammed" in the end.

This has major potential as a race car, "off-road use only" application.

By having load control be entirely electronic, I am free to further experiment with other things later. In the immediate term I won't need a throttle, a blow off valve, or any sort of idle control valve. I could try incorporating a smooth traction control, where the mixture is leaned out rather than having a rougher fuel or ignition cut as traditionally used. I probably won't need a wastegate on the second stage turbo of the compound setup, rather I could just stop injecting more fuel past a certain boost level, and it will lean out a bit (and cool the exhaust), and the boost will stop rising.

 

[pbloore]

Great Stuff!.

I keep thinking there's got to be a better way to run a spark ignition engine.

With respect to your misfire you might find a benefit from using individual injectors although there may be issues with the range of fuel flow available to cover idle to full power.

It would be interesting to see what the combustion variability is under these super lean conditions - that's one area where CI engines score over SI.

Keep up the good work.

Paul

 

[pbloore]

Just read your first note properly and realised you're using port injection!. Out of curiosity what injectors are you using, and will they cover your min to max fuel demands or are you using two injectors per port.

Paul

 

[obanion]

The injectors haven't been succesfully tested yet, but my plan is this.

550cc/min injectors (rathed for gasoline at 43.5psi). I figured they will flow far more with 120psi and liquid propane, enough to cover my fuel demands.

They are low impedance, and a side flow design. I'm hoping the side flow design, which has the pressurized fluid moving around the main body of the injector, will prevent injector failure usually associated with high pressure. I'll be sure and update here after I can test them.

 

[franzh]

I've discussed this many times, but when handling liquid propane in the injection phase, please be overly cautious! Liquid expands 270 times to vapor. Also, if the liquid is trapped in the injector lines, it can VERY quickly expand past the pressure limits of the injectors, rails, lines, and valves.
You will quickly encounter differences in fuel handling between the torch in vapor phase, and liquid phase. Remember that when shutting off the engine, the injectors will absorb heat and pass it through to the injectors. I have seen fuel rail pressures exceeding 400 psig, due to heat saturation. I have also seen the aftermath of blown up injectors and fuel lines.
Again, be cautious. Use due diligence and VERY thorough and sound engineering when attempting liquid phase injection, not to even get into the throttleless design.
Franz

 

[obanion]

The torch was only as a test. It was using vapor. I won't be using it for any sort of liquid phase injection.

Help me understand this high pressure in the lines problem you mention. I thought about it and I don't quite get how the problem occurs.

So you have some heat get in, and it raises the temperature of the liquid propane. Some of it boils, but as it does it does two things. Expand, which would increase pressure, which just happens prevents further boiling. It also cools, would would also, stop the boiling. To get to 400psi, the propane itself has to be at over 200F, or it would be a liquid.

I should note, it will be a very cool injection in operation. It will be mounted in a intake manifold on a I-6 engine, so exhaust heat is on the other side. The exhaust header and exhaust will all be fully heat insulated. The car will have a heavily vented hood. The intake manifold will be fully insulated from the engine compartment (encased in low heat conduction plastic shell), and be insulated from the head with a phenolic spacer. And, the propane will be cooling off most right at the base of the intake manifold, so if anything under normal operation the propane will be getting cooled, not heated.

As I see it, if you keep heat away from the propane, which I will, there isn't a problem.

So the problem would only lie on shutdown, and heat soak, though minor it would be in my case. Since I won't need nor use a check valve of any sort in between the tank and the rail, any expansion or boiling at the rail will just displace back into the tank, where it will cool and recondense.....I think. Then, next time the engine is started, the vapor in the line will purge through the injectors almost instantly. I imagine this approach is no good for widespread use, as a tiny amount of propane is likely to go out the tailpipe unburned as a result, but fine by me.

 

[pbloore]

For safety I would use at least a tank solenoid to shut down the fuel line if say the ignition is killed.

From an injector point of view I would echo Franz's comments - I don't think they will survive too long - apart from the pressure, the lubricity of liquid propane is significantly worse than gasoline.

I think Tickford did a liquid propane injection system for the European Ford Focus and Transit vehicles but had problems with the injection control particularly when regulating small amounts of fluid such as at idle. They also had issues with ice forming in the intake. You might be able to find some data from these programmes to help you.

As an alternative you could look at using vapour injection which would probably make idle and part load a lot simpler to regulate. There are quite a number of vapour phase injectors on the market, you would need a vaporiser in the system though which would complicate things.

 

[franzh]

Obanion:Lets try to wrap this up simply:
Propane is a pressurized liquefied vapor. The vapor pressure index describes what pressure is required to keep propane a liquid. This information is available in most engineering handbooks, especially ones that deal with LPG, and in NFPA 58.
Lets assume the tank temperature is 70 deg F. Your pressure will be around 120 psig. At 100 deg F, it would be around 190 psig. At 50 deg F, it would be around 85 psig.
LPG will indeed boil, but if the line is liquid saturated, and closed at both ends, it will expand until the line or injectors rupture. It will not auto-refrigerate unless the vapor can be released to a larger space. Also, if the fuel returns to the tank, it will expand, but it will not condense and refrigerate, just transfer the heat from a small area to a larger area (First and Second laws). I have seen liquid injection fuel tanks hot enough to blister your hands! ! !
This is the primary problem with the production LPG liquid injection systems. Tank pressures approaching the pressure relief range is not uncommon. Hot engine restart is difficult.
Restarting the engine, purging the vapor in the line (you stated you have a return loop, which fuel is pulled, vapor or liquid? You will not be able to operate the liquid injectors with the fuel map in vapor phase (270 times reduction in volume) and vice versa.
Pbloore comments are also valid.

Now, if you were just using vapor injection. . . .your biggest problem is sourcing injectors. Bosch and Delco both make CNG injectors, I would start with Bosch.
Regards;
Franz
ps: I would be happy to mail you a vapor pressure graph, send me your request to:
franzh "the @ symbol" texas.net

 

[obanion]

I'd like to use vapor injection, the problem being I can't find anything that will approach the flow requirements I need. The vapor injectors available only go up to about 25% of what I need, not even close.

I considered solenoids of some sort, but haven't found any that can open and close as fast as injectors. If anyone knows of some solenoids that might fit the bill, I'm eager to see them.

 

[pbloore]

I'm using a vapour phase injection system on my 4.6l V8. While it's a fairly mildly tuned engine, the injectors work very well. I'll see if I can look the specs up tomorrow, but they're Keihin injectors and from memory they're low impedance (peak and hold type) and rated up to about 28kW/cylinder depending on the vaporiser. This type of system is used in the UK on some quite high performance BMWs although not on any race engines to my knowledge. If you're doing a one off bespoke system you could always double up on injectors.

The system I'm using is manufactured by BRC who have a tie up with Impco, so I would have thought the bits should be available in the USA.

I think you'll find a vapour system a lot easier.

Paul

 

[obanion]

That would be great. So far, EVERY company that advertises having propane technology, vapor or liquid, refuses to sell me ANYTHING. I have asked about 5 companies at this point, and the answer always is "we won't sell to you." It's frustrating to know the parts I need exist, but are deliberately kept from being available.

One thought, vapor propane flow through any metering devise goes up significantly with pressure, because in addition to the sqrt(P1/P2) difference, you also have the propane becoming denser with pressure, which liquid fuels don't do. So if the vapor injectors you are using can handle it, I could just get by with more pressure.

 

[patprimmer]

Obanion

Find out who they will sell to and why, like spare parts for instance. You might need a specific application for the spares.

You might need an LPG fitters licence of some sort, or a friendly fitter as co-conspirator, then tell them what they want to, or need to hear. They may have legal restraints re safety, licences and liability, and a major fear of lawsuits, or criminal action. If you can eliminate these fears by taking the risk yourself, they may get a somewhat more willing.

I should not advise you to lie, but I am sure, that if there is a will there is a way, and you will figure it out

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[pbloore]

I haven't managed to find any detailed specifications of the vapour injectors I'm using but according to the installer notes there are two LPG vapourisers - a 1.2 and a 1.5 bar - and two injectors with different flow rates. The 1.5 bar vapouriser with four of the higher flow injectors is useable on four cylinder engines up to 140kW. For higher output engines two vapourisers can be used in parallel.

The injectors themselves are rated at up to 30 kW/cylinder natuarlly aspirated or 38 kW/cylinder turbo. I would have thought therefore that you could certainly use an injection pressure of between 1.5 and 1.9 bar without any problem. This should give you a capability of about 300 HP on a V6 with a single injector per cylinder and two vapourisers.

Regards

Paul

 

[obanion]

Ah, since I plan to run up to 4bar (absolute) boost pressure, I'd need to run the injectors as high as at least 5-6bar at that boost pressure. If they can handle it, I'd think the propane being 4x denser at 6bar than at 1.5bar, they could flow up to 900-1000HP for six.

 

[franzh]

Obanion;
You need to keep track of the vapor pressures and the Joules Thompson effect. How do you plan on supplying the varying pressures depending on boost levels?

In the US, many suppliers will not sell these specialized products to the general public due to liabilties. I have a close working relationship with many of them and I have to jump through hoops to obtain certain components. I often find it easier to go overseas and get what I need, but frequently lose some technical advantage then.
Many overseas manufacturers deal with smaller engines, thus smaller orifice sizing and flows.
Franz

 

[obanion]

Well I had one idea today. Maybe this will work.

160lb/hr injectors, by my calculations (software SF Pressure Drop), will each flow aprox 25-28lb/hr of vapor propane at 80psi (95psi absolute) rail pressure. That's enough for starting, idling, cruise, and light acceleration (less than 10psi boost).

I could have a sealed stainless pressure tank, pressurized by the same regulated pressure of the supply (80psi). This would drain out of the bottom of the tank, and lead to a solenoid, which is then connected right at the inlet of the fuel rail. At the flip of a switch, a solenoid in the propane line will close, and the solenoid for the methanol will open. The propane vapor will quickly purge out of the rail, and it becomes a methanol fuel system. I'd also have the fuel computer cut the duty cycle as well, as I'll need only about 40% as much duty to get the same lambda AFR with liquid 80psi methanol, compared to vapor 80psi propane.

I'd think, methanol would make more power than propane anyway, and be a better fuel for full power boost. I'd probably run with 5-10% water content as well.

Propane is a great fuel for all the other conditions (starting, idling, cruising, light acceleration), and I should still be able to run throttleless that way. The other bonus is I won't need to purge the fuel system of methanol after use, the propane will do that the instant I switch the solenoids back again.

In terms of controlling power when in methanol mode without a throttle, and I don't think I'd want to let it run lean, I was thinking of having the same ~25% fuel cut I'd use to maintain propane idle speed, also come into effect if I back out of the throttle at full boost. Throttle position drops below 90%, instigate a 25% fuel cut (meaning 1 out of 4 injection pulses), throttle position drops below 60%, cut fuel altogether.