Engines Output example essay topic

The engine tested was a Crossley Gas Engine, which when tested had a power output of 3.993 kW in comparison to this a typical petrol engine would have an output of approximately 70 kW and a tractor unit (used to pull an articulated lorry) would have an output 250 kW. As can be seen this is a large difference in power output however many factors must be observed to see why this is the outcome. Such factors which effect this is that the Crossley gas engine is an old single cylinder engine which and a speed of 181 rpm, to compare this to the speed of a typical petrol engine, which is around 6000 rpm and a tractor unit (diesel, compression ignition engine) of 1700 rpm. The Torque-speed characteristics of an engine are very important, this is because in an engine the higher the torque produced the higher the output of power, this can be seen as brake power is a function of torque (BP = Tfc). This means that if at a low engine speed the torque is large, then the power output will also be large and therefore a greater driving force will be achieved and thus a better engine is produced. It also stands to reason that if the same torque and therefore brake power can be produced at a lower engine speed the wear on the engine will be less than that of an engine which runs at a higher speed to produce the same outcome in torque.

The Crossley gas engine had a calculated thermal efficiency of 11.9%, where as a modern petrol engine would have an efficiency of around 25% and a diesel engine, such as that in tractor unit may have an efficiency up to 35%. The difference in these efficiency can be related to the fact that the Crossley gas engine has a low compression ratio (4: 1) where as the modern car engine has a ratio of 10: 1 and a tractor unit of between 20: 1 to 30: 1. The power output would vary with a change in the air / fuel ratio, but this would only be more! yen visible! | when the engine is working at full output. This is due to the fact that the limiting factor on an engine is the air entering the cylinder, since at full output all of the air is being burned, this mean that if the ratio of the fuel mixture is change to allow more air in then a higher output can be reached.

A P-V diagram would change with load because the area under the graph is represented by work done and the work done would also be dependant on the load and if the load was changed then the P-V diagram would also change. When a car is at rest but with the engine running there is fuel consumed but no actual power output to the drive wheels, if the car then starts to accelerate it will start to use more petrol but, there will be an output which is visible (as the car is moving). Then when the car reaches its optimum speed (an load) the petrol which is being used to keep the output at such a level that the car is at its optimum speed will be less than it would need to keep it at a lower speed, and so it can be said as the load increases the specific fuel consumption decreases. The volumetric efficiency is important to an engine because if the efficiency decreases as the size of the engine is increased then if a large engine was built for industrial use it would not be classed as useful if it could not maintain a reasonable efficiency.

In a modern car the energy output between the drive shaft, coolant (radiated heat) and exhaust gases are 1/3, 1/3, 1/3 respectively. Manufactures must ensure that the energy outputs from a car do not change so that a greater amount of exhaust gases are produced because they have to abide by law which state car must not produce a large amount of exhaust gases. If a greater proportion of energy is going to either coolant or exhaust gases, this also means there has to be less energy going to the drive shaft, and thus general efficiency of the car decreases. The Crossley gas engine does not have the same ratio as this. The main reason for this is the fact that it has a! yen hit and miss! | governor, which means that the fuel mixture may not always be ignited. The compression ratio, is the ratio at which the air is compressed in comparison to that of its original volume, this affects the performance as if more air can be compressed in the cylinder then the pressure will be increased and so when ignited the temperature will be higher and thus the volume greater, which increases the engines output.

On a car there is only a certain compression ratio which a cylinder can achieve, and so if a turbo charger is used then the compression ratio will be increased as a turbo charger injects more air into the cylinder. The main gases which are expelled from the exhaust are CO 2, CO, sulphur, lead and nitrogen. These gases are all dangerous and affect the levels of! yen smog! | in cities and also affect the ozone layer as CO 2 is a green house gas. This means that laws have been brought in by the government to restrict manufacturers into producing low emission cars. The government also try to promote people into buying cars with smaller engines, by reducing tax, as they produce fewer emissions. Measures can also be taken by drivers to reduce pollution.

If a car is running at constant speed then the exhaust gases are less than if revving or accelerating. Fitting a catalytic converter also converts some exhaust gases so that gases are emitted.