Last reviewed 11 February 2014
When selecting vehicles, transport managers should give consideration to both the type of engine and the available fuels. In this article, Richard Smith describes the various engine technologies available; a future article will look at the factors relating to the different types of fuel, including problems with bio-additives.
Choice of fuel type is a feature of the vehicle selection process but, at least in the case of commercial vehicles, it is probably taken as given that the answer will be diesel. That may well be right, but nevertheless, this is an issue that deserves more than just a cursory glance.
While the selection process is often thought of as a choice of fuel, it is really a choice of the type of power unit, as that is what governs the fuel that can be used. In addition, a decision based solely on the relative merits of different fuels will ignore some important factors that may carry greater weight. So, this part of the vehicle selection process really needs to run on two parallel lines: one involving the characteristics of different types of power unit; and one involving the different types of fuel. A trade-off may well have to be made at the end of the exercise.
The table below shows two columns, one detailing the available types of power unit, the other the various fuels. It should be noted that there is not necessarily a direct correlation between the power unit in the first column and the fuel in the second. This article is mainly concerned with the characteristics of the different power units.
Compressed natural gas (CNG)
Liquefied petroleum gas (LPG)
The spark ignition (SI) engine compresses a mixture of volatile fuel and air in the cylinder, and then ignites the mixture by means of an electric arc at the precise required time. The combustion of the fuel results in an increase in the pressure in the cylinder, which forces the piston down to turn the crankshaft. Provided that the amount of fuel and the time of the ignition are closely controlled, the increase in pressure is not damaging and almost complete combustion results, with little or no pollutant products. The maximum torque (“pulling power”) is generated at mid-range engine speeds. Modern SI power units have a life in excess of 100,000 miles with regular servicing. SI power units require a highly refined gasoline fuel that is volatile and highly flammable, although specially converted or designed units can also operate on LPG or CNG.
Compression ignition (CI) engines operate in the same way as spark ignitions, with the essential difference that it is air alone that is compressed in the cylinder, and combustion is initiated spontaneously by injecting a very fine spray of fuel into the compressed (and therefore hot) air at the required time.
This method of combustion requires the air to be compressed to a much greater extent than in the SI engine and when combustion occurs, the pressure generated in the cylinder is therefore much higher. This means that the engine must be heavier in construction. The pressure under which the fuel is injected also has to be much higher, meaning the injection system has to be more robust, with the internal components made to much finer tolerances.
These factors increase the cost of the basic power plant over that of the equivalent SI one, and it is generally noisier. Engine life is generally longer than for SIs, and fuel consumption is usually better, although servicing costs may be higher. Greater torque is generated at lower speeds, giving the CI engine greater load-hauling capability. The engine is, however, quite sensitive to fuel quality and adjustment of the injection timing and, if these are not ideal, damaging and noisy shock waves are created with an increase in polluting emissions and noise.
Also note that a basic problem with this type of combustion is the production of particles of soot, even under the best conditions, and these are of great environmental concern.
CI engines generally run on a refined petroleum fuel oil known as diesel engined road vehicle (DERV) fuel that is less volatile and flammable than gasoline. Alternatives can be produced from plant sources (bio-diesel), and CI engines can also be built or adapted to run on LPG or CNG.
Electric motors are quiet and produce no pollution when in use. They also generate their maximum torque at zero revs, giving greater acceleration from standstill.
Servicing requirements for the motor are minimal, but the downfall is that a substantial battery is required to provide the power for the motor. This battery is heavy, expensive and causes great environmental impact in both production and disposal at the end of its relatively short life. Even with modern advances in battery technology, the range it provides for a vehicle is still short (reduced still further by higher load and speed) and it takes a long time to recharge it.
The cost of fuel saved is replaced by the cost of electricity required to charge the battery, and the pollution emission is merely transferred from the vehicle to the power station if fossil fuel is used to generate the electricity. Full electric power may be viable for small vans operating over short ranges in urban areas, but this is not yet an option for longer distance and heavier vehicles.
A hybrid power unit is simply a combination of a spark or compression ignition engine and an electric motor. The two can work together in a variety of ways, such as:
with the engine acting purely to generate electricity to charge the battery, which then powers the electric motor to drive the vehicle (series hybrid)
with the electric motor used to supplement a smaller engine when extra power is required for hill climbing or acceleration (parallel hybrid)
with the electric motor alone being used at low speed, and both power units together for maximum acceleration — between these two states the engine is used on its own and any excess power goes to charge the battery (parallel hybrid).
The battery can also be charged by the energy generated when the vehicle slows down (regenerative braking) and sometimes also by plugging in to the mains. Hybrid power units reduce fuel consumption and polluting emissions, but there is the extra weight and cost of the battery to factor in. For its 26-tonne FE Hybrid, Volvo claims reduced noise and carbon dioxide emissions, and fuel savings of up to 20% on a typical distribution profile.
A gas turbine is basically a jet engine that drives an output shaft rather than providing thrust through a stream of hot gasses. Gas turbines have been used to power experimental road vehicles for more than 50 years and several manufacturers have produced prototype heavy goods vehicles, although there are not thought to be any in current production. Turbines are smoother than piston engines, can run on a wide range of fuels with a high power-to-weight ratio, reduced fuel consumption and maintenance, and significantly lower nitrogen oxide and particulate emissions.
The vehicle selection process should include a thorough consideration of both the type of engine and the available fuels. As previously mentioned, a future article will look at the factors relating to the different types of fuel, including problems with bio-additives.