It has long been known that work can be turned into heat, and the petrol engine is a good example of the reverse process which consists in turning heat into work. In a steam engine and boiler plant the heat of the fuel is liberated under the boiler, and then a portion of it gets transferred to the water in the boiler and forms steam, which is then taken to the engine and does work in the cylinder, the whole being a wasteful process. The petrol engine is an internal combustion engine, or one in which the fuel is burnt inside the engine cylinder itself and converted directly into work. From every British Thermal Unit of heat liberated by the combustion of the fuel in the cylinder we should be able to get 778 foot-pounds of work if the thermal (or heat) efficiency of the engine was 100 per cent. The thermal efficiency (η) of any engine may be defined as the ratio which the heat equivalent of the work done per minute by the engine bears to the heat which would be liberated by the complete combustion of the quantity of fuel admitted to the cylinder per minute. Thus—
η = ((Horse-power of the Engine × 33,000)/778)/((Number of pounds of fuel consumed per minute) × (Calorific Value of the fuel))
Example:—An engine developing 30 horse-power uses 0·50 lb. of benzol per minute. What is its thermal efficiency? The calorific value of benzol may be taken as 19,000 B. Th. U. per lb.
η = (30 × 33,000/778)/(0·50 × 19,000) = 0·134, or 13·4 per cent.
[APPENDIX]
ENGINE TROUBLES
Many of the troubles that are likely to arise have already been referred to in previous chapters, but the following additional notes may be found useful.
1. Engine refuses to start.
Care must be taken to observe exactly what happens, and one cannot do better than ask oneself mentally some of the following questions.
(a) Is the ignition “on”?