Rethinking the Diesel Engine

More developments lead to fewer emissions

August 1, 2009

The environmental demands on truck manufacturers are becoming steadily tougher, and this autumn European emission standards will become tougher still.

Pressure for the development of environmentally optimized technology is increasing, but the answer need not lie solely with electric and hybrid solutions. A century-old technology may, in fact, be the key as we seek out tomorrow’s energy-efficient engines. Some indicators are pointing to the diesel engine as the future.

In 1892 Rudolf Diesel was probably unaware that his invention, the “Neue, rationelle Wärmekraftmaschine” (new efficient thermal engine), would have an impact not just at the time but decades into the future. Behind patent number 67207 lay a combustion engine that was far more efficient than any of the steam- or gas-powered engines that were common at the end of the 19th century.

Today the diesel engine is seen as one of the most promising technologies as we continue the hunt for new engines for an increasingly eco-aware and resource-efficient world. The reason for the diesel engine’s success lies in Diesel’s original idea: the creation of an engine with the maximum thermodynamic efficiency rating – something that is achieved when as much as possible of the fuel’s energy is used to propel the vehicle instead of literally going up in smoke.

What makes this power plant special is that it compresses both air and fuel under high pressure. When the fuel in the cylinder ignites owing to the heat generated by the high degree of compression, this generates the power that sets the piston in motion.

Five years after its introduction, the diesel engine boasted an efficiency rating of 26 percent, well past the 12 percent efficiency of the contemporary steam engine. Comparably, modern diesel engines output an efficiency of about 46 percent. A conventional gasoline engine, for instance, today offers a maximum efficiency rating of about 40 percent.

So what exactly has happened over these past decades?

“The diesel engine’s development can be divided into three stages,” explained Maximilian Prager, an expert in combustion engines at the Technical University of Munich. “The first step was the rapid development of the fuel injection system which took place in the early 1900s. Around 1940 system pressure had increased from 300 bar to more than 700 bar, which meant combustion was both quicker and more efficient.”

The creation of new, lightweight air compressors was a much needed second step in the equation. With the mechanical supercharger, and later the turbine-propelled turbocharger, air pressure could be doubled from 2 bar to more than 4 bar, a step that further improved the engine’s efficiency.

“The most recent major step came with the development of the electronic common rail injection system at the start of the 1990s. This builds up a kind of pressure reserve that can be used to inject fuel several times per operating cycle. This both raises the power output of the engine and reduces emissions,” said Prager.

All these improvements meant that the diesel engine could strengthen its lead over other engine types.

Fuel Efficiency

“In terms of the number of liters per kilometer, the diesel engine is about 20 percent more efficient than a petrol engine. For one thing, you can get 10 percent more energy out of a liter of diesel than from a liter of petrol, and for another, the diesel engine operates about 10 percent more efficiently than the petrol engine does,” said Prager.

Surprisingly, this energy efficiency puts the diesel engine in the lead even when it comes to emissions. Diesel engines have been long regarded as “dirty” because they have a tendency to emit visible smoke consisting of unburned carbon. Now thanks to the development of particle filters, these emissions have been cut. And with the latest SCR (selective catalytic reduction) technology, emissions of nitrogen oxides also have been reduced.

One Example

Examples of how the diesel engine has been improved over the years can be found, for instance, in Volvo trucks. According to Volvo, over the last 20 years the company’s engines have become 100 times cleaner in terms of emissions of climate-impacting air pollutants. In addition, power output has increased by 100 percent in 35 years.

Through optimized combustion and improved exhaust filtration, engines have become both more fuel-efficient and cleaner than ever.

But if all this is already history, what can we say about the diesel engine’s future?

“The next 10 to 15 years will be tough for engine developers as regards the increasingly stringent exhaust emission requirements,” said Prager.

Immense potential exists for new diesel technologies that are currently being developed, he added.

Biofuels can help cut emissions, and hybrid diesels with single-point technology in which the diesel engine is optimized solely for long-distance operation, for instance, may promote both higher efficiency and lower production costs in the future.

“There is immense potential in the diesel engine, primarily for large engines which have to work both hard and long. The diesel engine’s high efficiency rating makes it quite simply indispensable in the development of tomorrow’s engines,” said Prager.