Derek,
All carbureted engines use a heated plenum of some kind to improve
vaporization, so there is some basis for this idea. However, I'm dubious
that there is much improvement to be gained over that provided by a properly
operating intake manifold heater (normally heated by deflected exhaust
gasses). The following I thought was an interesting analysis of engine
efficiency. Note the use of heated gasoline and air to increase thermal
efficiency to 32%, which is high for any gasoline engine.
There are many variables that will determine the power output of an engine.
High on the list will be the ability of the fuel to burn evenly without
knock. No matter how clever the engine, the engine power output limit is
determined by the fuel it is designed to use, not the amount of oxygen
stuffed into the cylinder and compressed. Modern engines designs and
gasolines are intended to reduce the emission of undesirable exhaust
pollutants, consequently engine performance is mainly constrained by the
fuel available.
A Honda Civic uses 91 RON fuel, but the Honda Formula 1 turbocharged 1.5
litre engine was only permitted to operate on 102 Research Octane fuel, and
had limits placed on the amount of fuel it could use during a race, the
maximum boost of the turbochargers was specified, as was an additional
40kg penalty weight. Standard 102 RON gasoline would be about 96 (R+M)/2 if
sold as a pump gasoline. The normally-aspirated 3.0 litre engines could use
unlimited amounts of 102RON fuel. The F1 race duration is 305 km or 2 hours,
and it's perhaps worth remembering that Indy cars then ran at 7.3 psi boost.
Engine Standard Formula One Formula One
Year 1986 1987 1989
Size 1.5 litre 1.5 litre 1.5 litre
Cylinders 4 6 6
Aspiration normal turbo turbo
Maximum Boost - 58 psi 36.3 psi
Maximum Fuel - 200 litres 150 litres
Fuel 91 RON 102 RON 102 RON
Horsepower @ rpm 92 @ 6000 994 @ 12000 610 @ 12500
Torque (lb-ft @ rpm) 89 @ 4500 490 @ 9750 280 @ 10000
The details of the transition from Standard to Formula 1, without
considering engine materials, are:-
1. Replace the exhaust system. HP and torque both climb to 100.
2. Double the rpm while improving breathing, you now have 200hp
but still only about 100lb-ft of torque.
3. Boost it to 58psi - which equals four such engines, so you have
1000hp and 500lb-ft of torque.
Simple?, not with 102 RON fuel, the engine/fuel combination would knock
the engine into pieces, so....
4. Lower the compression ratio to 7.4:1, and the higher rpm is a
big advantage - there is much less time for the end gases to
ignite and cause detonation.
5. Optimise engine design. 80 degree bank angles V for aerodynamic
reasons, and go to six cylinders = V-6
6. Cool the air. The compression of 70F air at 14.7psi to 72.7psi
raises its temperature to 377F. The turbos churn the air, and
although they are about 75% efficient, the air is now at 479F.
The huge intercoolers could reduce the air to 97F, but that
was too low to properly vaporise the fuel.
7. Bypass the intercoolers to maintain 104F.
8. Change the air-fuel ratio to 23% richer than stoichiometric
to reduce combustion temperature.
9. Change to 84:16 toluene/heptane fuel - which complies with the
102 RON requirement, but is harder to vaporise.
10.Add sophisticated electronic timing and engine management controls
to ensure reliable combustion with no detonation.
You now have a six-cylinder, 1.5 litre, 1000hp Honda Civic.
For subsequent years the restrictions were even more severe, 150 litres
and 36.3 maximum boost, in a still vain attempt to give the 3 litre,
normally-aspirated engines a chance. Obviously Honda took advantage
of the reduced boost by increasing CR to 9.4:1, and only going to 15%
rich air-fuel ratio. They then developed an economy mode that involved
heating the liquid fuel to 180F to improve vaporisation, and increased
the air temp to 158F, and leaned out the air-fuel ratio to just 2% rich.
The engine output dropped to 610hp @ 12,500 ( from 685hp @ 12,500 and
about 312 lbs-ft of torque @ 10,000 rpm ), but 32% of the energy in
the fuel was converted to mechanical work. The engine still had crisp
throttle response, and still beat the normally aspirated engines that
did not have the fuel limitation. So turbos were banned. No other
F1 racing engine has ever come close to converting 32% of the fuel
energy into work
Bob Palmer
rpalmer@ucsd.edu
rpalmer@brobeck.com
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