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# 4 Petrol and diesel engines

The vast majority of the world’s road vehicles are powered by petrol and diesel internal combustion engine (ICE), so we will start by looking at this technology, the emissions involved and ways that these emissions could be reduced.

## Petrol engines

The most common form of petrol engine is the four-stroke Otto cycle engine. This uses a piston which is driven up and down inside a cylinder and connected to the drive section by a rotating crankshaft. At the top of a four-stroke engine there is a cylinder head containing a number of valves controlling the flow of gas in and out. The four ‘strokes’ are: induction, compression, power and exhaust, illustrated in Figure 8.

Figure 8: The four strokes of an Otto cycle engine

On the induction stroke a small amount of fuel and air is drawn into the cylinder through the open inlet valve. The fuel/air ratio has to be controlled very tightly (typically to 1 gram of fuel to 14.7 grams of air) for clean combustion. In older cars the fuel and air was mixed in a carburettor, but in modern ones a specified quantity of fuel is injected by a computerised engine management system. When the piston reaches the bottom of the stroke the inlet valve closes. On the next stroke this air/fuel mixture is then compressed into typically one-tenth of its original volume, creating a highly inflammable mixture which is then ignited using an electric spark on a sparking plug. The gases then burn very rapidly reaching a high temperature (750 °C or more) and expand, pushing down the piston on the power stroke. Finally, on the exhaust stroke, the burned gases are pushed out into the exhaust system through the open exhaust valve. The whole cycle then repeats.

The power of the engine is controlled by varying the total amount of fuel and air admitted to the cylinder.

The reduction in volume during the second stroke is a rather critical factor called the compression ratio. If the volume of the cylinder is 300 cc when the piston is at the bottom of its stroke and the mixture is compressed down to only 30 cc when the piston is right at the top, then the compression ratio is 300:30 or 10:1. Typical figures for modern car engines are between 9:1 and 13:1.

An animated version of this four-stroke engine figure can be found on the Animated Engines website.

## Diesel engines

The diesel engine works using the same four-stroke cycle as the petrol engine, but with two major differences involving the air–fuel mixture and injection systems. In the diesel engine, only the air is compressed in the cylinder instead of an air–fuel mixture, and at the end of the compression stroke, the fuel is directly injected into the combustion chamber by a fuel injection pump. In modern engines this pump is likely to be controlled by a computerised engine management system.

A typical compression ratio of 20:1 is used, which is sufficient to raise the air temperature to over 400 °C. Once the diesel fuel is injected into the cylinder, it immediately vaporises and spontaneously ignites without needing a sparking plug.

The power of the engine is controlled by varying the quantity of fuel injected. This means that the fuel/air ratio can vary over a wide range.

This diesel engine cycle can also be seen on the Animated Engines website

In general, the fuel efficiency of a diesel engine is higher than that of a petrol engine. This is primarily due to the fact that the combustion temperature within a diesel engine is higher (higher temperatures give a higher Carnot efficiency).

The higher combustion temperature also leads to higher emissions of nitrogen oxides (discussed later).

Petrol fuel and diesel for road vehicles (DERV) are internationally traded standard commodities (even though they are sold by different companies at the pump). The low heating value (LHV) properties of these fuelsare shown here in Table 2. (These values assume that the water vapour produced in combustion is released into the exhaust as steam).

## Table 2 Properties of DERV and petrol

| Low heating value /MJ kg−1 | Low heating value /kWh litre−1 | LHV CO2 emissions /g CO2 MJ−1 | LHV CO2 emissions /kg litre−1
--- | --- | --- | --- | ---
DERV (no blended biofuel) | 42.8 | 10 | 74 | 2.7
Petrol (no blended biofuel) | 44.8 | 9.1 | 70 | 2.3

BEIS, 2020
Note that although diesel fuel has a slightly lower energy content per kilogram than petrol, it is denser and so contains more energy per litre. Diesel fuel contains a higher proportion of carbon and thus has a higher (i.e. worse) CO2 emission factor than petrol.

## Petrol and diesel vehicle energy efficiency

In a diesel car engine about 32% of the heat energy is delivered to the crankshaft, compared to only about 24% in a petrol engine. As this energy is delivered to the wheels via the mechanical drive-train, more energy is ‘lost’ owing to friction. As a result, in theory about 24% of diesel fuel’s energy ends up being used for moving the car; in the case of petrol this figure is only 18%. In practice, the actual values found vary enormously with the vehicle type and with the driving conditions. ICEs are particularly inefficient in slow stop/start urban motoring and in situations of high acceleration; they work most efficiently running at a constant speed (for example on motorways).

If we consider how much of the fuel’s energy is actually used to move the payload (as opposed to the whole vehicle), the situation is even worse. Only around 1–2% of the fuel’s energy is used to move the vehicle’s occupants.

## Box 2 Miles per gallon and litres per 100 km

The traditional UK unit of vehicle fuel efficiency is ‘miles per gallon’. This course uses the metric unit of ‘litres per 100 km’.

An old petrol car might achieve a consumption of 10 litres per 100 km. What is that in miles per gallon?

There are 4.546 litres in a (UK) gallon and 1.609 kilometres in a mile.

10 litres per 100 km = 10 / 4.546 gallons per 100 km

= 10 / (4.546 × 100) gallons per km

= 10 × 1.609 / (4.546 × 100) gallons per mile

= (4.546 × 100) / (1.609 × 10) miles per gallon = 28.3 miles per gallon

More simply:

Miles per gallon = 283 / litres per 100 km

Litres per 100 km = 283 / miles per gallon

## Activity 2

10 minutes

List the key differences between petrol and diesel engines.

(A text entry box would appear here, but your browser does not support it.)

### Answer

Petrol engines:

- compression ratio around 9:1 (up to 13:1 max.)
- relatively low temperature and pressure
- tightly controlled fuel/air ratio
- overall efficiency in moving the car about 18%

Diesel engines:

- compression ratio around 20:1
- direct injection of fuel
- variable fuel/air ratio
- relatively high temperature and pressure
- overall efficiency in moving the car about 24%

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