HP turbo LPG engine: Advice re Fuelling Ignition, & CR 4

[ralphw]

Hello from the UK

I have I several questions that I would appreciate some help with. The context for all these questions is my personal car which has a turbo charged 2 litre 4 cylinder 16 valve petrol engine. The engine has done 65k miles, of which 45k are subsequent to an LPG conversion (Vapour phase, sequential injection). It produced 285.9 bhp when tested soon after conversion in Jan 2007 so it has a reasonably high specific power output.

1. Fuelling and ignition timing strategy for high load operation.
In the UK similar cars regularly use pump fuel, RON 99, MON ~86 and can operate with manifold boost pressures of ~1.4bar. Knock is controlled via the duel strategy of rich fuelling and retardation of ignition timing.

However for an LPG engine I am concerned that the traditional gasoline approaches to fuelling/ignition appear compromised. LPG as dry fuel cannot contribute evaporative charge cooling and excess fuel can only add inert thermal mass to the charge. Also I have read that LPG has a slower flame speed than an equivalent gasoline /air mixture under similar combustion chamber conditions so retarding the ignition risks overheating exhaust valves. So what approach should we use to optimise the high load LPG fuelling and ignition timing? (The fuel and ignition maps can be altered relatively easily using aftermarket ECU reflashing software and separate gasoline and LPG maps can be stored and switched into use as required).

2. Selecting the best compromise Compression Ratio / peak boost to optimally exploit the raised LPG octane.
I am contemplating several changes to the engine specification, one aspect of which is an increase in compression ratio. I am primarily motivated to improve both the off boost driving pleasure and the cruise efficiency but am struggling to provide a rational basis for any CR increase. I have a fuel which is several points higher in Octane and I want to get the best out of it both at cruise and high load.

As a benchmark for comparison I have started with the assumption that product of boost pressure (absolute), compression ratio and a function to include the adiabatic temperature rise across the entire pressure ratio of the engine combine to a constant indicative of the "conditions of combustion" Combining this approach with an anecdotal rule of thumb relating octane to boost "2 octane points per psi of bost increase" and I estimate that a CR in the range 9.5 to 10 :1 might be plausible with the current levels of boost. Does this seem why off the mark? Comments, suggestions, some solid theory, all gratefully received.

3. Lubrication/ accelerated cylinder head wear with LPG
Vapour phase propane (I assume) has next to no lubricating properties. Is there any evidence to suggest that vapour phase injection can cause accelerated inlet valve to valve guide wear. Recently my engine has developed a cold start problem. Compression cranking and leak down tests suggest that the compression is adequate but an inlet manifold vacuum test indicates that the idle vacuum, particularly when cold, is less than it should be. The LPG installer (who is also a specialist mechanic for my make of car) has seen a couple of previous examples of similar configuration but normally aspirated engines which have presented with worn inlet valve/valve guides subsequent to an LPG conversion. Does this seem plausible?

Ralph

 

[MastDesgin]

The engine information is important, make, model, year, all modifications including turbo size (impeller and compressor size), amount of boost your currently running, stock or aftermarket cams, and stock or aftermarket ignition system, cooling system upgrades, head and valve modifications. Basically if your going to ask such a complex question you have to give complex details.

 

[bcs5274]

lpg & cng engines use specific valves and guides for the "dry" fuels, converting a gas engines usually has valve and seat wear that is higher than normal

 

[ralphw]

Ok , I thought my questions were reasonably generic but if the exact spec of my car is of interest or necessary here we go:
Subaru WRX MY MY2004
• standard 2.0 litre engine and TD04 turbo. I don't yet know exact compressor or turbine spec, and in any case the turbo can be treated as a design variable as far as I am concerned . I suspect we are “wringing the neck” of the current turbo but I’ve stuck with it since this is a daily drive car
• standard 380cc gasoline injectors at standard OEM line pressure
• Standard cooling system
• Exhaust, 3 cats removed, replaced by Prodrive down pipe sports cat. Prodrive 2.5in back box. (about as far as one can go and still maintain some decorum!!) exhaust manifold ported to minimised steps between built up cast manifold sections
• Standard inlet hose (widely perceived as a poor item) replaced by aftermarket SAMCO item (which to the naked eye appears to offer better flow characteristics). Standard Airbox and filter retained
• Ecu remaped including boost increase from 1 to1.4 bar , some ignition advance added for LPG. Full load AFR measured at ~12:1 using wide band lambda sensor calibrated for Gasoline, (For LPG there is a small discrepancy away from Stoichiometric due to the altered C: H ratio I believe).
• LPG conversion using LANDI vapour phase injector kit. We’ve doubled up on the spec using 2x injectors per port and 2x PRV since we estimated that a 1x install would not have delivered sufficient fuel.
Since the lpg conversion 45k miles ago the engine had been run almost exclusively on lpg, except for start up when a coolant temperature of 50C is required before the lpg fuel is switched on.

With regard to the comments by bcs5274: I’m contemplating a new engine build so using valves/guides with superior material properties more suited to lpg dry fuelling would be something I be very keen to adopt if it were possible. Does anyone have any more information the dry fuel / valve / valve guide topic? We’re contemplating using some form of Phosphor Bronze as a guide matl. Good Idea/Bad idea/no difference?


One final comment, I'm sorry if this thread is seen as complex but of all fora I'd have thought this was the one place where complex questions were acceptable? I made some effort to word it appropriately, perhaps next time I'll post 3 distinct threads, but this was my 1st effort here, I do apologize!!

 

[YvesLLewelyn]

Might be simpler to go back to petrol. Why do you want to use LPG? There is a lot of LPG use in Oz where it is about half the price of petrol - but I don't think it has any real advantages otherwise.

 

[ralphw]

In th euk lpg is less than half the price of premium unleaded... its a no brainer. I can actually enjoy driving the car without worrying about fuel costs. Plus I want to solve the engineering issues, that's why I'm an engineer!

 

[YvesLLewelyn]

I didn't realize LPG was cheap in the UK - I don't think it is in the US.

 

[patprimmer]

I thought your OP was reasonably generic and detailed where necessary for a good answer from someone who knws this subject.

Low price for high octane is certainly my interest in LPG turbo, especially the concept of regaining lost chemical energy with higher boost due to higher octane.

Regards
Pat
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[franzh]

In my LPG career, I have never seen inlet valve seat or guide wear attributable to LPG. The exhaust valve, however, does show significant wear. This is especially true if low compression or retarded timing is used, or if timing is not remapped to fully utilize the combustion properties of LPG.
Many tuners just add 10° of timing to what is already there and that can really screw things up. If you have access to a rolling road dyno and can remap the timing on the fly, set timing to mean best torque for the timing available, before the onset of detonation.
Also, on more than one project, I actually shifted the AF ratio lean at max boost to control detonation versus a rich shift, especially since as you stated, there is relatively little liquid mass to absorb head for evaporation. It worked and actually served as a boost reduction method as well. In this case, increasing AF ratio or retarding the timing actually increased exhasut temp and in one case caused a jump in boost pressure.

Franz

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[ralphw]

Franz

When you say you leaned the AFR on boost do you mean less rich but still rich (Lambda<1) or do you mean properly weak lambda >1!!. To the petrol turbo guys rich high boost fuelling is a mantra. I've seen a guy pull an EVO off an rolling road test because it was running Lambda 0.9 on boost instead of the obligatory 0.8 or less. so if your proposing a weak lpg AFR on boost, this is radical!!

And another question, given optimal timing and fueling, does lpg burn hotter or colder than petrol both at high and low loads?

why is low compression linked to exhaust valve damage... lower pressure -> even slower burn??

thanks Ralph

 

[Lou Scannon]

The main reason lower compression increases exhaust gas temperatures is that the expansion ratio is lower.
Power enrichment provides internal (evaporative) cooling with liquid fuels but does very little that is beneficial with gaseous fuels. With gaseous fuels a better option for "internal cooling" is excess air.

 

[mattsooty]

Hemi,

Agreed, quite often a gaseous engine will knock as its AFR approaches stoich, from the lean side, and the response is then excess air.

Admittedly a bit of a funny one for the gasoline cal engineer in me but sort of makes sense from the diesel perspective when excess air is used to reduce exh temps.

MS

 

[ralphw]

Hmm!

Do the above comments apply to high pressure turbo gaseous engines? Are you guys saying that correct fuelling strategy would be Stoic at low load, leaning at high load using excess air as the det inhibitor?? Does anyone have any experience that this works (with a boost say 1-1.4 barg), I find it difficult to imagine that I'd ever have the courage to try it!! it is so anti the "perceived wisdom".

Also how much excess air are we talking about? The raison d'etre of an HP turbo engine is to cram all that air in and then use it all up, not cram it in and use some of it....

 

[franzh]

I've been off the net for a couple of days but Hemi and mattsooty are both correct.

One Cummins 5.9 I played with had severe detonation at 8 psig at stoich. The client didnt want to install a wastegate on the original turbo. We tried retarding the timing but it only led to excessive exhaust temps. Leaning the fuel mixture from 1.0 to 1.5 killed the boost and the detonation (and the power). The lean feature was triggered by a pressure switch set at 4 psig. It worked.

Franz

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[Bribyk]

With industrial gaseous-fueled engines you typically lean out the mixture to avoid detonation. Check out the link for a pretty handy (if general) diagram. It is for natural gas so it will be similar, just your detonation zone will be lower on the chart (as propane is more prone to detonation than methane).

http://www.cumminspower.com/

http://www/literature/technicalpapers/PT-7009-LeanBurn-en.pdf

Brian Bobyk - Hoerbiger Canada

 

[ralphw]

Ouch!! If a lambda of 1.5 -1.7 is what it takes to gain control of the combustion then this is not a viable option for me. I plan to maintain or improve the full load power, not cut it ~33%!!. Of course the Subaru head is likely to offer considerably differing susceptibility to detonation so a direct comparison to my engine is probably unrealistic. I would hope and expect the Scooby head to perform better?

The real answer is LPPFI, wins all ways round except that the technology still appears experimental and it’s considerably more expensive, but I digress.

Is there any evidence to suggest a dual fuelled approach offers any benefit. I currently need x2 lpg injectors but for the majority of daily load conditions the engine would be adequately fuelled by a single injector per cylinder. Similarly, with regard to the petrol fuelling the injector duty cycle is close to it’s ceiling at full power, but an increase in injector size reduces the precision for cruise fuel ling (I presume). So why not cruise with lpg running stoic or lean then add gasoline “richly” at high load?
The hope would be that there would be that evaporative cooling from the gasoline would help to control detonation. A very subtle balancing act this one I suspect. Has it been tried? One obvious problem is the cross over if the lpg is fuelled weak in the first instance........ Perhaps a better implementation would be an efficiency mode which runs lpg only, uses a single injector, excess air det control. And a Performance mode with dual fuel that uses rich fuelling as the det control throughout the load range.... Now that might work??

Thanks for all the comments guys!

 

[Bribyk]

You can keep the same power (and get better efficiency) at those lean air fuel ratios, you add more air not cut back fuel.

Brian Bobyk - Hoerbiger Canada

 

[YvesLLewelyn]

I doubt if a normal car engine (unlike an industrial engine) would even run at all at lambda 1.5/1.7.
I may be old-fashioned but to me even slightly lean mixtures mean poor performance and very possible engine damage.

 

[ralphw]

In the car world adding the air is the hard part. Maybe in an industrial engine you can readily add the extra air but it's not so straight forward for an automobile.

I made a search on lean burn and I found this, you might recognize the author, thanks to franzh

http://franzh.home.texas.net/lean.html

This raises some questions concerned with the transients encountered during acceleration. My understanding of conventional gasoline fueling is that comprises an essentially static correlation between injector duration, charge load and engine rpm. However to accomodate a lean burn philosphy it would seem neccesary to have an active fueling which would enrich to enable vehicle acceleration but weaken during "cruise conditions" Is this correct?
Can a dual state system, i.e. either lean map or enriched map be made to work or is a more continuously variable "dynamic adaption" required?
How does the turbo boost control sit with lean burn?

 

[Lou Scannon]

The concepts we're discussing (lean-burn, turbocharged gaseous fueled engines have been around for several decades, first in stationary industrial engines, and more recently (since mid-90's or so) in on-highway bus & truck engines.
The turbo boost control doesn't care what lambda is, it is up to the engine management system to simultaneously control boost and lambda.
You use the term "enriched" which has validity in this discussion, but be advised that the air fuel ratio map in the type of engine we're discussing is a continuum from "richer" (but still lean of stoich) at idle to "leaner" at full load(anywhere from lambda 1.4 to lambda 1.7 or so, depending on the engine and its application).
In some cases these engine management systems have transient fueling strategies that may enrich (i.e. make less lean, not go rich of stoich) the mixture temporarily to assist with transient performance, but I don't think this is too common especially on on-highway applications, since this strategy will increase NOx emissions in the EPA and Euro transient test cycles. Stationary engines typically aren't subject to transient emissions tests so such a strategy can be used without penalty. Since significant speed and/or load transitions tend to be relatively infrequent in stationary applications, the incremental NOx emissions aren't necessarily that signficant in this case.