LPG Camshaft and Mixer Selection

[JTP173]

I have been discussing the idea of running wider lobe separation angles for LPG/Propane engines than you would on an equivalent Petrol/Gasoline engine with my colleagues and I am interested in hearing your ideas. If you agree that wider separation angles should be used to what extent do you think they should be widened?

Also does anybody know the minimum pressure difference required across Impco mixers, such as the 225 or 300a, in order for them to work correctly?

 

[JTP173]

I forgot to ask about peoples thoughts on exhaust to inlet ratio (cylinder head flow) when using LPG/Propane.

 

[Lou Scannon]

I can't think of a fundamental reason why LSA (lobe separation angle) should be altered when going from a liquid to a gaseous fuel. However, when going from sequential (timed) port injection to pre-mixed single-point metering, there is definitely an argument for reduced valve overlap (in order to reduce loss of fuel to the exhaust during the overlap phase). Increased LSA will reduce overlap, everything else being equal.

 

[franzh]

I agree. If the engine is intended for mild street use where reversion or intake pulses create problems with cylinder fuel distribution, increasing the LSA will help. The higher the rpm or if the engine is going to be used in a more aggressive mode, the camshaft selection is less critical.

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[JTP173]

You agree with hemi you mean?

I understand what altering the lobe separation angle does, that was not my question, but thanks anyway.

So neither of you think that the possible improvements in cylinder filling when running wider lobe separation angles are important? It is beneficial when a system is under carbed, which, unless you want to increase port size, is to an extent what happens when running LPG.
As well as the above, running wider lobe separation makes engines less sensitive to slightly incorrect component selection, which is likely when building an LPG engine as information is limited and most engines are built using an understanding of petrol engines.
There is also the added benefit of improved idle quality and vacuum, which is important when using any non-injection based gas system (not a big deal with small cams).
I thought somebody may have studied the influence of varying the lobe separation in LPG engines as people have done with Methanol (which uses different values to petrol). I haven’t done any such study, however, I have heard often from engine builders that LPG requires wider angles; unfortunately, they are unable to give me a reason why.

Can you help me with my other questions?

 

[franzh]

The majority of the cams that are ground specifically for a gaseous fuel engine focus on exhaust valve performance. Since the gaseous fuel tends to heat the exhaust valve and seat more than a liquid fuel engine, emphasis is placed on keeping the valve seated as long as possible for heat transfer.
Many HD engine builders that work with gaseous fuels prefer a wider exhaust valve seat and face than standard, and others set the valve lash as loose as possible to provide the longest on-seat duration as practical.
Realize that these specially ground cams make up only a tiny fraction of the LPG or CNG powered vehicle market. In the vast majority of the vehicles, the standard camshaft works just fine, including two of my vehicles that ran well over 200,000 miles with no problems.
For any performance issues, my personal thoughts are that the traditional aftermarket cams work just fine, just be prepared for some exhaust valve service.

Franz


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[JTP173]

Thanks mate. You have a website with LPG information with component selection haven't you? If so can you provide me with a link that will get me to the information as the one I tried didn't work.

 

[franzh]

I pulled it and condensed it in a book I sell (this is not the proper place) which covers most items in a propane engines.

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[patprimmer]

Franz

I think it is legitimate to post a link to a website here.

If that website listed your book, no problem.

I would also be interested to buy a copy

Regards
Pat
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