ralphw
Nuclear
- Mar 7, 2010
- 9
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
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
