Energy Efficiency of Vehicles  Print page
Fig. 1   In Hong Kong, buses, light buses and trucks usually use diesel engines.
Fig. 2   A petrol engine under the hood of a private car

Most vehicles on the road are powered by internal combustion engines. The most common internal combustion engines for vehicles are fuelled with petrol or diesel. In Hong Kong, private cars are mostly powered by petrol internal combustion engines, while commercial vehicles e.g. buses and trucks are driven by diesel internal combustion engines. Although both engines have similar structures, they have significant differences in terms of operation, energy efficiency, and the amounts and types of emission they produce.

Petrol and diesel engines

There are two main differences between the design of a petrol engine and a diesel engine. First, petrol engines use spark ignition while diesel engines use compression ignition. Inside an engine, fuel is burned to release and convert the chemical energy to kinetic energy. In a petrol engine (Fig. 3), a spark plug produces sparks to ignite the mixture of petrol and air. In a diesel engine (Fig. 4), air is compressed to a high temperature so that when a fine mist of diesel is pumped into the compressed air, the intense heat causes the diesel to ignite. This is why diesel engine is called a compression ignition engine.

Fig. 3   The structure of a petrol engine Fig. 4   The structure of a diesel engine

The second difference is the range of compression ratios of the engines (see the animations below to understand the meaning of "compression ratio"). Diesel engines are designed to operate at higher compression ratios, usually between 15 to 20, while petrol engines have lower compression ratios usually between 8 to 10.Since the higher the compression ratio of an engine the higher the thermal efficiency, so diesel engines generally have a better fuel efficiency than petrol engines. To power a similar sized vehicle, a diesel engine will be about 15% - 25 % better on fuel efficiency than a petrol engine [1].

But diesel engines also have disadvantages. Although diesel engines are more energy efficient, they produce significantly more particulates in the exhaust gas than petrol engines. Diesel engines are also slightly noisier and less refined. To withstand the high compression ratio, diesel engines are more expensive to produce. These factors make diesel engines more suitable for commercial vehicles, because these vehicles usually cover longer distances each day, making diesel engines' fuel efficiency advantage important.

Take a look at the following two animations on a petrol engine and a diesel engine to understand more about their operations.

Flash animation: Inside a petrol engine

Flash animation: Inside a diesel engine

Fuel consumption of cars

In order for a car to move forward, the chemical energy of the fuel is converted to kinetic energy for the car. The fuel also has to supply energy to cover energy losses due to friction and air resistance, for example. When an engine is in operation, there are many parts moving up and down or rotating. As long as the engine is running, even when a car is stopped (idling), some engine parts are moving and energy is being consumed. When a car is under heavy acceleration, all the moving engine parts have to move faster in order to deliver more power to the car, more fuel is consumed as a result. This is why hard acceleration is less energy efficient than smooth acceleration. Travelling at high speeds is also less energy efficient as the main energy losses due to friction and air resistance increase with speed.

Fig. 5   Smaller cars generally consume less fuel than larger cars, especially when travelling at low speed.

Heavier vehicles consume more fuel than lighter ones. Recall that the kinetic energy of an object is given by mv2 / 2, so it requires more energy to bring a car with a larger mass m to a certain speed v. When driving at low speeds, vehicles fitted with larger engines are also less energy efficient. A larger engine is more powerful, but it needs more energy to move heavier engine parts and to cover higher frictional loss. These energy losses become significant in proportion to the useful kinetic energy of the car when the car is travelling at low speeds.

The energy label for petrol passenger cars

Fig. 6   EMSD's document on fuel saving tips for private cars

The Electrical and Mechanical Services Department (EMSD) has established the Energy Efficiency Labelling Scheme for Petrol Passenger Cars [2]. The energy label provides information on urban fuel consumption, highway fuel consumption and estimated annual fuel consumption, based on standardised fuel consumption tests for easy comparison between vehicles. With the information provided, car buyers can compare the fuel consumption of different cars before making a decision. EMSD has a web page showing the fuel consumption data for vehicles that have participated in the scheme [3].

Tips for car owners

Private car owners can make their driving more energy efficient. The government provides tips on how to reduce fuel consumption and emissions. By following these tips, we can make car driving more environmentally friendly. Click on the following activity to find out what these tips are.

Activity: Fuel efficiency tips for cars

Mass transportation

The development of a convenient mass transportation system in Hong Kong aims at providing for the transportation need of Hong Kong, and to reduce traffic congestion, energy consumption and pollution.

Fig. 7   A double-decker bus Fig. 8   The KCR Man On Shan Rail

Mass transportation is more energy efficient and produces less pollution than small scale transportation. Take the comparison between a double-decker bus and a private car as an example. An average private car has a mass of about 1500 kg and a capacity of 5 people. A double-decker bus has a mass around 15,000 kg and a capacity of about 120 people. A private car moves 300 kg of the car's mass per passenger while a double-decker bus moves 125 kg of the bus' mass per passenger. Moreover, the total energy loss due to friction of many private cars added together is larger than that of a single double-decker bus.

Table 1 below shows that in 1997, mass transportation, including the railways (MTR and KCR) and buses, produced much less carbon dioxide, nitrogen oxide and RSP (respirable suspended particulates) than cars and taxis. This means that mass transportation is more environmentally friendly.

Table 1 Emissions of different modes to transport obtained using data from Annual Traffic Census 1997
Mode Sub-category Carbon dioxide (CO2) Nitrogen oxide (NOx) Respirable Suspended Particulates (RSP)
Rail Weighted average 46.57 0.21 0.007
Road Cars 213.92 1.25 0.029
Taxis 147.57 0.82 0.0342
Public light bus 38.02 0.19 0.061
Single-decker bus 76.99 0.87 0.107
Double-decker bus 35.81 0.27 0.033
Weighted average 83.07 0.54 0.082
(in units of grams per passenger per kilometre)
Source: The Second Railway Development Study Final SEA Report - Annexe B [4]

Various measures have been implemented to encourage the use of mass transportation, improve fuel efficiency and reduce pollution. These include the use of alternative fuel vehicles e.g. LPG (liquefied petroleum gas) vehicles to replace diesel vehicles; the introduction of Bus-bus Interchange Schemes, and Park-and-ride Schemes.

Liquefied petroleum gas (LPG) vehicles

Liquefied petroleum gas vehicles e.g. LPG public light buses and LPG taxis produce much less pollution than diesel vehicles. Table 2 shows the results of a comparison of diesel public light buses with LPG light buses. You can see that LPG produces much lower emissions and particulates. This is a major improvement compared to diesel.

Fig. 9   Virtually all taxis in Hong Kong are now LPG taxis. Fig. 10   The LPG tank on a taxi

Table 2 Test results comparing diesel with LPG public light buses
Mode Pollution Reduction Percentage of LPG Light Bus Compared with Euro III Diesel Model
Hydrocarbon + Oxides of Nitrogen 51%
Carbon Monoxide 87%
Particulate Very low emission from LPG Model
Smoke No black smoke from LPG Model
Source: Report of the Alternative Fuel Light Bus Trial, Appendix 11, Monitoring Committee of the Alternative Fuel Light
Bus Trial [5]

The Hong Kong government plans on replacing all diesel taxis with LPG taxis by the end of 2005. By the end of 2003, about 18,094 LPG taxis were operating on the roads, representing over 99% of the whole fleet. The replacement of diesel taxis with LPG taxis helps improve the air quality in Hong Kong.

Promoting mass transportation

The Bus-bus Interchange Scheme involves establishment of interchange bus stops with fare rebates or fare waivers provided for passengers to change to another bus route. This encourages the public to use buses for long distance travel, and at the same time reduces the need for long distance point to point bus routes. This further increases the efficiency of buses as part of the mass transportation system in Hong Kong.

Fig. 11   The Bus-bus Interchange at Shing Mun Tunnel Fig. 12   Many bus routes are available free of charge for passengers to change to at the Shing Mun Tunnel Interchange

Another scheme that promotes the use of mass transportation is the Park-and-ride Scheme. This scheme aims at encouraging the use of railway resources by commuters who normally travel by private cars to urban areas. Parking facilities are provided at strategic railway stations for people to drive to the railway station, park their car and complete their trip to urban areas by train. This scheme encourages the use of the more environmentally friendly and energy efficient railway system instead of driving to urban areas, and it should help improve energy efficiency and air quality.

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