On Tue, 06 Jan 2004 21:39:11 +0000, Stewart
wrote:

Because there's more to the internet than hits alone, Peter Hill
wrote:

As the UK infrastructure is in place now is the time to
move to 100% LPG fueled vehicles with OEM optimised engines and do
away with the dual fuel conversions.

Well I'm not going to hold me breath.

Once we start to get engines designed around the fuel, then it seems
liquid injection looses much of it's point, with not much, if any,
advantage over simpler gaseous injection.

Without liquid injection the compression can't be raised to take
advantage of the octane rating.

A change of compression ratio from 9.3:1 to 12:1 increases efficiency
by 12% (it also gives more power). Each increase in octane by 6 will
allow an increase of 1 in compression ratio. Petrol is 95 octane,
Propane is 111, this should allow the engine to run 12:1. BUT only if
the inlet temp stays the same. If inlet temp goes up you can't
achieve the same gain and an inlet temp rise (by feeding it hot vapor)
completely negates the possibility of any gains.

A normal petrol engine has it's inlet temp reduced by about 20 deg C
due to vaporisation of the fuel. Every 4 deg C lower inlet temp
results in 1% efficiency gain (hot vapour feed results in 5% loss!).
With liquid propane this reduction in inlet temperature will be closer
to 30 deg C (2.5% efficiency gain) as the LPG will vaporise completely
unlike petrol which goes in as an atomized mist and some vapor. This
reduced inlet temp will allow a further slight increase in compression
= a bit more efficiency.

Liquid propane injection should allow an increase of about 15% on
existing engine inefficiencies. So an engine that today runs at 33%
on petrol should give close to 38% efficiency when optimized for
liquid propane. This ignores the improved combustion efficiency due
to complete vaporisation - could give over 40%! A gain of just 9% to
36% would still make it worthwhile.

A hot propane vapor injected engine looses about 5%, it regains some
due to better combustion efficiency but at best can only just match
the petrol engine.

Propane burns cooler than petrol (1925 vis 2030 deg C) but propane
vapor engines run hotter due to loss of cylinder head and valve
cooling (as demonstrated by some engines valve seat problems). Liquid
injection gives higher efficiency = more power, better fuel
consumption, lower engine temps, longer life ..... but it costs more
and can't be designed and fitted by plumbers so the cheap option wins.

--
Peter Hill
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