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Testing Engines

Before any engine is put on the market, it has to be thoroughly tested.  Nobody wants an engine that is going to fail in use (there is nothing more useless than a broken-down car).  So various tests are done on engines, of which we will look at a few.

Useful Power

You will have seen that many engines have their power quoted as brake horsepower (bhp).  This has been used by engineers for at least two hundred years.  At its crudest, it is a comparison with the power you can get out of a horse, which had been the common form of motive power for many centuries.  However a more scientific test was needed, and the diagram below shows the kind of set up, called a dynanometer.

The mass and strap act as a brake because they provide a frictional couple, T (= mgr) against the rotation of the engine.  The power produced by the engine is given by the formula:

P = Tw

[P - power (W); T - torque (Nm); w - angular velocity (rad/s)]

Originally 1 brake horsepower worked out at 746 W; now it is considered as 750 W.  It is often given the shorthand PS ("Pferdstärke", German for "horsepower").

Question 7 An engine gives out a torque of 250 Nm at 3300 rpm.  What is its power in watts and PS?  ANSWER

The method above, although simple, has a disadvantage in that a lot of heat is generated.  Although the principle is much the same, the test beds for engines are much more sophisticated.  They can be:

• hydraulic with the engine driving a pump;

• electrical with the engine driving a generator into a load.

The picture below shows an engine test bed:

This picture shows a water pump that an aircraft engine manufacturer uses to test engines before they are reinstalled into aircraft.  It mimics the loads experienced by the engines in flight.

The useful power that can be got from an engine is always less that the power worked out from an indicator diagram.  This is because there is friction within the engine.  The power needed to overcome friction is the friction power:

friction power = indicated power - brake power

Question 8  In Question 6 you worked out that the indicated power of an engine was 5700 W.  The power available at the output shaft is 4.7 kW.  What is the power dissipated in overcoming frictional losses in the engine?  What fraction is this of the indicated power?  ANSWER

The answer you worked out in the previous question shows that a lot of power is used to overcome friction.  It is dissipated as heat.  Oil lubrication is essential in such an engine:

• It reduces friction;

• It takes away the heat produced by friction.

Without lubrication the engine would rapidly seize up.

Engine Efficiency

The indicated or thermal efficiency is given by:

thermal efficiency = indicated power ÷ power input from the fuel

As we have seen there are mechanical losses in an engine.  The mechanical efficiency of an engine can be defined as the ratio of the output power to the indicated power or workable power.  The output power is the power we can get from a dynanometer.

mechanical efficiency = output power ÷ indicated power

Question 9  What is the mechanical efficiency of the engine in Question 8?  ANSWER

As the engine runs faster, the power absorbed in overcoming friction increases, so the mechanical efficiency falls away.  We can see this in the graph below:

The frictional power increase almost mirrors the decrease in mechanical efficiency.

The overall efficiency is the fraction of the input power of the fuel that is delivered as useful power:

Overall efficiency = output power ÷ input power of the fuel.

Question 10  In Question 6 you worked out that the power gained from the fuel of the engine was 15.8 kW.  If the power output is 4.7 kW, what is the overall efficiency?  ANSWER

The overall efficiency of internal combustion engines is not very good, with even the best being about 40 %.

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