Internal Combustion Engines

Internal Combustion Engines

The internal combustion engine does away with the need for an external heat source.  Fuel is burned within the engine to provide the heat that does the useful work.  Generally these engines use fossil fuels which are particularly concentrated forms of energy.  We will look at the two most common types:

• The petrol engine which uses the Otto Cycle;
• The diesel engine.

The Otto Cycle

The four-stroke Otto cycle is shown in the diagram:

The indicator diagram for the Otto cycle is like this:

Let's look at the cycle and link it to the indicator diagram:

1. The induction stroke takes place at A.  Although in theory the pressure should be the same as atmospheric, in practice it's rather lower.  The amount of petrol air mixture taken in can be increased by use of a supercharger.
2. A to B is the compression stroke.  Both valves are closed.  The compression is adiabatic, and no heat enters or leaves the cylinder. 
3. Ignition occurs at C.  The gases resulting from the ignition expand adiabatically, leading to the power stroke.
4. D to A the gas is cooled instantaneously.  
5. At A the exhaust stroke occurs and the the gases are removed at constant pressure to the atmosphere.
6. Strange as it may seem, the piston does half a revolution at A.  Actually it's slightly in practice, as the the valve timing is more complex.

In practice the thermodynamics of a petrol engine are more complex:

• Fuel burns during the cycle, so the number of moles is not constant.
• The cycle takes place very quickly, so there is swirling of the gases.  The kinetic energy of gases is not taken into account in these indicator diagrams.
• There are considerable temperature gradients, so we cannot deal with the gas as if it were constant temperature.
• Ignition takes a finite time, and takes time to propagate through the fuel-air mix.  Therefore pressures will vary within the gas.

The efficiency of a petrol engine can be increased by increasing the compression ratio.  However the heating of the gases can ignite the petrol prematurely.  This pre-ignition is known as knocking or pinking.  It can do a lot of damage to the engine.

Diesel Cycle:

The Diesel cycle differs from the Otto cycle in that the induction stroke takes in only air.  The are is compressed quite a lot so that it gets hot.  The fuel is injected into the hot air, and ignites.  This produces the power stroke.

The indicator diagram is quite different to that of a petrol engine:

Let's now look what happens in the indicator diagram:

1. The induction stroke takes air in ideally at constant volume, pressure at temperature.
2. The compression stroke takes place from A to B.  The air is compressed adiabatically to about 1/20 of its original volume.  It gets hot.
3. From B to C fuel is injected in atomised form.  It burns steadily so that the pressure on the piston is constant.
4. From C to D the power stroke moves the piston down as adiabatic expansion takes place.
5. D to A cooling and exhaust occurs.

The diesel engine has a higher thermal efficiency than the petrol engine.  However it does have the disadvantage in that it is heavier.  Also the size of engine for a given power tends to be bigger.  They also tend to be noisier and incomplete combustion makes for considerable pollution.

However diesels have been made lighter and more refined for luxury cars.  Experiments with diesels for aircraft have been hugely successful.  Jet A1 fuel (paraffin) costs 30 p a litre compared with Avgas (unleaded aviation petrol) at 90 p a litre.

This aircraft uses two 1.7 litre diesels (of the same type as found in Mercedes cars, but with higher quality components).  It can fly at 360 km/h, and flying at 150 km/h burns about 3 litres of fuel per hour.  Rather more economical than a family saloon, but at 300 000 euros not exactly a snip.  The picture below shows the engine used, the Centurion 1.7

For either kind of engine, we can predict the power that the engine can give out by using a simple formula:

Power output = area of p-V loop x no of cylinders x number of cycles per second

A common bear trap is that a single cylinder four stroke engine goes through each cycle once every two revolutions. 

We can also work out the maximum energy that can be put into an engine by this formula:

Input Power = calorific value of fuel x flow rate of the fuel

The fuel for any engine has a calorific value which is the energy that can be got out of the fuel per unit mass.  It is measured in joules per kilogram.  For wood the calorific value is about 20 x 10⁶ J kg^-1, while for oil it is 42 x 10⁶ J kg^-1.

In engineering articles, watch out for fuel flows in kg min^-1 which need to be converted to kg/s. 

Question 6

Test-bed measurements made on a single-cylinder 4-stroke petrol engine produced the following data:

• mean temperature of gases in cylinder during combustion stroke 820 °C
• mean temperature of exhaust gases 77 °C
• area enclosed by indicator diagram loop 380J
• rotational speed of output shaft 1800 rev min^-1
• power developed by engine at output shaft 4.7kW
• calorific value of fuel 45 MJ kg^-1
• flow rate of fuel 2.1 × 10^-2 kg min^-1

(a) The rate at which energy is supplied to the engine

(b) The indicated power of the engine;

(c) The thermal efficiency of the engine.  (AQA Question, adapted)  ANSWER

Now go on to Testing Engines

The internal combustion engine does away with the need for an external heat source.  Fuel is burned within the engine to provide the heat that does the useful work.  Generally these engines use fossil fuels which are particularly concentrated forms of energy.  We will look at the two most common types:

• The petrol engine which uses the Otto Cycle;
• The diesel engine.

The Otto Cycle

The four-stroke Otto cycle is shown in the diagram:

The indicator diagram for the Otto cycle is like this:

Let's look at the cycle and link it to the indicator diagram:

1. The induction stroke takes place at A.  Although in theory the pressure should be the same as atmospheric, in practice it's rather lower.  The amount of petrol air mixture taken in can be increased by use of a supercharger.
2. A to B is the compression stroke.  Both valves are closed.  The compression is adiabatic, and no heat enters or leaves the cylinder. 
3. Ignition occurs at C.  The gases resulting from the ignition expand adiabatically, leading to the power stroke.
4. D to A the gas is cooled instantaneously.  
5. At A the exhaust stroke occurs and the the gases are removed at constant pressure to the atmosphere.
6. Strange as it may seem, the piston does half a revolution at A.  Actually it's slightly in practice, as the the valve timing is more complex.

In practice the thermodynamics of a petrol engine are more complex:

• Fuel burns during the cycle, so the number of moles is not constant.
• The cycle takes place very quickly, so there is swirling of the gases.  The kinetic energy of gases is not taken into account in these indicator diagrams.
• There are considerable temperature gradients, so we cannot deal with the gas as if it were constant temperature.
• Ignition takes a finite time, and takes time to propagate through the fuel-air mix.  Therefore pressures will vary within the gas.

The efficiency of a petrol engine can be increased by increasing the compression ratio.  However the heating of the gases can ignite the petrol prematurely.  This pre-ignition is known as knocking or pinking.  It can do a lot of damage to the engine.

Diesel Cycle:

The Diesel cycle differs from the Otto cycle in that the induction stroke takes in only air.  The are is compressed quite a lot so that it gets hot.  The fuel is injected into the hot air, and ignites.  This produces the power stroke.

The indicator diagram is quite different to that of a petrol engine:

Let's now look what happens in the indicator diagram:

1. The induction stroke takes air in ideally at constant volume, pressure at temperature.
2. The compression stroke takes place from A to B.  The air is compressed adiabatically to about 1/20 of its original volume.  It gets hot.
3. From B to C fuel is injected in atomised form.  It burns steadily so that the pressure on the piston is constant.
4. From C to D the power stroke moves the piston down as adiabatic expansion takes place.
5. D to A cooling and exhaust occurs.

The diesel engine has a higher thermal efficiency than the petrol engine.  However it does have the disadvantage in that it is heavier.  Also the size of engine for a given power tends to be bigger.  They also tend to be noisier and incomplete combustion makes for considerable pollution.

However diesels have been made lighter and more refined for luxury cars.  Experiments with diesels for aircraft have been hugely successful.  Jet A1 fuel (paraffin) costs 30 p a litre compared with Avgas (unleaded aviation petrol) at 90 p a litre.

This aircraft uses two 1.7 litre diesels (of the same type as found in Mercedes cars, but with higher quality components).  It can fly at 360 km/h, and flying at 150 km/h burns about 3 litres of fuel per hour.  Rather more economical than a family saloon, but at 300 000 euros not exactly a snip.  The picture below shows the engine used, the Centurion 1.7

For either kind of engine, we can predict the power that the engine can give out by using a simple formula:

Power output = area of p-V loop x no of cylinders x number of cycles per second

A common bear trap is that a single cylinder four stroke engine goes through each cycle once every two revolutions. 

We can also work out the maximum energy that can be put into an engine by this formula:

Input Power = calorific value of fuel x flow rate of the fuel

The fuel for any engine has a calorific value which is the energy that can be got out of the fuel per unit mass.  It is measured in joules per kilogram.  For wood the calorific value is about 20 x 10⁶ J kg^-1, while for oil it is 42 x 10⁶ J kg^-1.

In engineering articles, watch out for fuel flows in kg min^-1 which need to be converted to kg/s. 

Question 6

Test-bed measurements made on a single-cylinder 4-stroke petrol engine produced the following data:

• mean temperature of gases in cylinder during combustion stroke 820 °C
• mean temperature of exhaust gases 77 °C
• area enclosed by indicator diagram loop 380J
• rotational speed of output shaft 1800 rev min^-1
• power developed by engine at output shaft 4.7kW
• calorific value of fuel 45 MJ kg^-1
• flow rate of fuel 2.1 × 10^-2 kg min^-1

(a) The rate at which energy is supplied to the engine

(b) The indicated power of the engine;

(c) The thermal efficiency of the engine.  (AQA Question, adapted)  ANSWER

Now go on to Testing Engines