Saturday, Apr 02, 2016 at 13:03
The science of fuel combustion still confounds a lot of people and there are many people who have a poor understanding of it.
Higher octane (premium) petrol is designed for high-compression engines.
Petrol BURNS in an engines combustion chamber - it doesn't explode.
If petrol explodes in a combustion chamber - this is called pre-ignition or detonation - and it produces the classic "pinging", or light knocking engine sound, as it does so.
A higher octane rating means the fuel has more resistance to detonation - PULP actually BURNS more SLOWLY, than the lower octane ULP.
The additional POWER from premium fuels comes not only from the slightly higher calorific (or higher joule) value of the premium fuel - it also comes from the higher COMPRESSION RATIO of the engine - that is DESIGNED and OPTIMISED, to run on PULP.
Virtually every car engine today is fitted with a knock sensor (to prevent engine damage) - whereby the ECU will automatically retard the spark timing, to reduce or eliminate knocking or pinging.
To take full advantage of the benefits of PULP, the ignition needs to be advanced - or the ECU needs to be reprogrammed (reflashed) to tune the fuel mixture and ignition parameters to suit PULP.
When you use PULP in a small stationary engine that is designed to run on ULP - there's no ECU fitted, and there's no manual spark adjustment - so therefore there's no ability to advance the spark timing to take advantage of the higher octane rating of PULP.
Therefore, when using PULP in a small air-cooled engine, you have a fuel which is SLOWER BURNING - and this means that combustion temperature peak, from the combustion process, is now occurring in the exhaust port, rather than the combustion chamber.
As a result, you have a higher Exhaust Gas Temperature (EGT) - which leads to a burnt exhaust valve, if the high EGT continues for an extended period.
As anyone on here with aviation experience knows - on an aircraft piston engine, EGT is a crucial measurement that is a primary focus of
the pilot.
EGT is one of the most critical engine parameters that needs to be watched carefully.
Engine manufacturers will often give a maximum EGT figure for their engines and this must not be exceeded to prevent engine damage.
EGT is also a crucial number to extract the best fuel economy or power from an engine - and EGT is monitored by pilots, via the mixture control lever, to enrich the mixture to make full power for takeoff, and to lean the mixture to increase fuel economy, once cruise level is reached.
Adding extra fuel into the air-fuel mixture is utilised to cool an engine by reducing combustion temperatures.
This is the reason why all the big old American petrol engines were thirsty.
The manufacturers designed them to run on a rich Air-Fuel Ratio (AFR) to prevent exhaust valves from melting at constant highway speeds, as most American engines/cars were run at.
In a small air-cooled engine, cooling is carried out by the oil - by the piston and conrod - and by the finning on the head and upper cylinder area.
When you use PULP in a small air-cooled engine - which is optimised by the manufacturer to use ULP - then the later combustion temperature peak is reducing the ability of the piston and conrod and oil to carry away the heat of combustion - and transferring the temperature peak, to the exhaust port and exhaust valve.
As a result, due to the inability of the small air-cooled engine to reset its spark timing (either manually or automatically) - or to raise its compression ratio - to take advantage of the higher octane rating of PULP - you're wasting your money, gaining no extra power - and risking a burnt exhaust valve, by using PULP in it.
Sorry about the long post - but fuel combustion in an engine is a complex process that always involves a lot of factors, all of which need to be understood to get a grasp of the full picture.
Cheers, Ron.
AnswerID:
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