Modes of Transport - Road

The car has become the main means of passenger transport in most countries around the world. Road transport also dominates the freight market. The proportion of energy consumed in road transport is correspondingly high compared with the other modes of transport. Road transport is also expected to account for the greatest rates of increase in freight and passenger traffic - but this need not necessarily go hand in hand with a further increase in energy consumption. Improvements in conventional engine and vehicle technology and new propulsion systems can help to reduce the consumption of fossil fuels in particular and to save costs.

Conventional engine technology

Engine technology offers a major path towards improving energy efficiency in road transport and reducing fuel consumption. The direct-injection dual-charged internal combustion engine with high specific capacity and low consumption improves the energy efficiency of petroleum-fuelled engines by approx. 20%. Here, German firms can provide innovative world-beating solutions.

The principle of the diesel engine offers consumption-related advantages over the internal combustion engine. In the case of gas-fuelled engines, natural gas combustion processes offer consumption levels equivalent to those of diesel, coupled with minimal emissions.

Hybrid drive

Hybrid drive - whereby a vehicle is initially accelerated by an electrical motor before the diesel engine takes over - offers great promise for cars, urban buses, lift-trucks and local freight distribution. Also, in many cases the energy produced by braking is recovered and returned to a newly developed, high-performance, long-life and light lithium-ion battery or to the "ultra caps" for the electrical drive. And there is usually an automatic start/stop mechanism which switches off the engine during longer stand-still periods and starts it up again when the accelerator is lightly pressed or a gear is engaged. In the case of urban buses, hybrid drives can cut fuel consumption by 20 - 25% compared with diesel-only vehicles. A German manufacturer recently launched the world's most efficient luxury hybrid limousine. Its petrol consumption is a mere 7.9 l per 100 km, resulting in the world's lowest CO2 emissions for this vehicle and power class, of only 186 g per kilometre. The power output is 220 kW/299 bhp, and the combined maximum torque 385 Nm. The hybrid luxury limousine undercuts vehicles of this class with conventional engines by up to 2.1 l per 100 km.

Electrical drive

Vehicles which power their propulsion from an electrical energy storage facility are also an energy-efficient alternative for road transport. Electrical motors are less complex than combustion-based engines, need a less complex gear system and produce zero local exhaust emissions when in operation. There are two types of electrical motor in use in road transport-battery-powered and fuel-cell.

The key to the success of battery-powered vehicles lies in the battery technology. The Li-ion-based batteries currently used for IT offer great potential for use in vehicles. They can store substantial quantities of energy in a reasonable size and weight, thereby fulfilling an important precondition. Ranges of 200 km and more are possible from Li-ion batteries in a compact class car. It is thus already worthwhile using battery-powered electrical vehicles, particularly in urban areas.

Another approach is to use fuel cells in vehicles. The chief advantages of the fuel cell are the level of efficiency, which is roughly twice that of the combustion engine, and the zero-emission operation. In a fuel cell, "cold combustion" of hydrogen and oxygen transforms chemical energy into electrical energy highly efficiently. This can be used both to power the vehicle via an electrical motor and to operate the numerous other appliances.

The focus of the development of both of these types of engine is currently on increasing the range and on reducing the price of the vehicles. One priority here is to optimise the performance of the on-board storage system.


In addition to new engine technologies, there are a number of further ways to improve energy efficiency in road transport. In the case of the HGV, for example, aerodynamics offer particular potential. Taking all the aerodynamic possibilities for the tractor unit and the trailer together, the drag co-efficient (cw figure) can be cut by around 20% and consumption by approx. 3 l per 100 km, despite the fact that full cladding including complete covering of the wheel arches adds at least 220 kg to the weight. German superstructure manufacturers recently presented a vehicle with a payload of 1,100 kg and a maximum permissible gross weight of 3.5 t which - thanks to aerodynamic improvements - consumes 4 l diesel less per 100 km than conventional transporters. The higher purchase price should be recouped after roughly two years.

Lightweight construction

High-strength steel, aluminium, or carbon composites - lightweight construction is a well-tried means used by German car and truck designers to cut consumption or increase the payload by reducing the unladen weight, thus reducing the consumption per tonne of payload. The lightest superstructure developed so far by a German manufacturer weights 5,400 kg. Conventional superstructures weigh roughly 1,000 kg more. The weight reduction means that an extra 1,000 kg of freight can be transported, thus saving journeys and, in terms of the weight transported, fuel costs.

Increasing capacity

EuroCombis - extremely long trucks - are extremely efficient. Field tests using vehicle combinations up to 25.25 m in length and up to the currently permissible weight have shown that 15 - 30% less fuel is consumed per transported tonne. Two EuroCombis can carry the load of three current trucks, which means that the volume of traffic can be substantially reduced. German-manufactured trailers lead the world and are therefore exported around the world.

Training the driver

Training the driver can cut truck fuel consumption by around 5% in the long term. If a training course is assumed to cost around €370 per driver, plus the loss of the services of the driver during the course and staff costs, the expense of the training course will have been recouped within two to three months. Sensitive driving also produces substantial savings in terms of wear-and-tear on tyres, brakes and clutch, as well as a much lower damage rate. Further to this, German firms manufacture intelligent driver assistance systems which keep the driver constantly informed about his driving and provide tips on improvements. If proper use is made of these systems, fuel savings of between 2.5 and 5% can be achieved, depending on the load and the distance.


Downsizing is an effective way to cut fuel consumption and emissions of pollutants for both internal combustion and diesel engines. Small cylinder capacities are used to generate high performance and torque. Downsizing therefore necessitates particularly efficient fuel injection and supercharging systems and makes great demands of motor engineering.

Downsizing can make a higher utilisation of the existing engine capacity possible. The selection and combination of appropriate technologies in terms of cylinder capacity, power and torque can produce substantial fuel consumption reductions of 10 - 30% compared with traditional engine concepts. Basically, there are two ways to downsize: increasing the output of an existing engine by supercharging or by increasing rpm, or replacing a large engine with a small engine with the same nominal power. The scale of the downsizing frequently depends on the relationship between the required transport performance and the desired reduction in fuel consumption.