Things that are Handy to Know

Things that are Handy to Know

The term 'brake horsepower' is a measure of the power delivered at the engine output shaft; measured by means of a dynamometer or similar braking device. The term 'shaft horsepower' [shp] is a measure of the engine power available at the propeller shaft. Generally it is the same as bhp but if the coupling is not direct drive — a propeller speed reduction unit [PSRU] is interposed between the crankshaft output and the propeller shaft as in the Rotax 912 — the shp will be a little less than bhp because of the power loss in driving the belt or gear driven PSRU.

The use of the horsepower term for piston aero engines has successfully withstood metrication. To convert horsepower to watts multiply by 745.7 or by 0.75 to convert to kilowatts. When torque is expressed in newton metres, and engine speed in radians per second, power will be in watts.

The stoichiometric (chemically correct) air/fuel mixture produces complete combustion of all the fuel and all the oxygen in the cylinder charge — and also the highest temperatures, which may be detrimental to the engine metallurgy. The stoichiometric air/fuel ratio for gasoline fuels is 14.7:1 by weight.

Spark ignition engines provide best power with an air deficiency of 5–15% from stoichiometric — i.e. about 12–13:1 (rich) — and provide minimum fuel consumption with around 10% excess air; i.e. about 16:1 (lean).
This indicates that the engine, at sea-level and using a stoichiometric mixture, would process about 8500 litres of air per litre of fuel. (Avgas weighs 0.71 kg per litre, and air at sea-level weighs 1.225 kg per 1000 litres.) The leaned mixture for best economy cruise is around 16:1 (9000 litres of air), and for maximum engine rich mixture performance, around 12:1 (7000 litres of air).
The Rotax 912 1.2 litre engine produces 75% power at 5000 rpm, and with a firing cycle every second revolution it would process 1.2 x 5000/2 = 3000 litres of air/fuel mixture per minute. The fuel used would be 3000/9000 = 0.33 litres/minute or around 20 litres/hour, at sea-level.

Most four-stroke, normally aspirated, aero-engines between 80 and 400 hp have a specific fuel consumption close to 0.19 kg or 0.27 litres, per horsepower per hour (or 0.42 lbs/hp/hr). Then the Jabiru's engine, rated at 80 hp, but using only 65% for the 97 knot cruise, would consume 80 × 0.65 × 0.27 = 14 litres over 100 air nautical miles, or 7 air nautical miles per litre. Note that you can create a little rule of thumb here that is applicable to most four-stroke engines — "the fuel burn, at 'performance cruise speed', is about one-fifth of the rated engine horsepower — in litres per hour." Thus, fuel burn for the Jabiru cruising at 75% power is 80/5 = 16 litres/hour. Two-stroke engines have to use a richer mixture to run cooler so, for such engines, add about 10% to the calculated result.

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