Why we still need the diesel engine

This year is the 80th birthday of the car diesel engine. Yet a look at the engineering principles shows they are very far from being obsolete. Even its critics have to concede: without diesel engines, the EU goal of reducing climate-damaging carbon dioxide emissions to 95 g/km by 2020 is barely achievable. What’s more, the technologies also exist for curbing nitrogen oxide emissions from diesel engines to a currently acceptable level.

In fact, the diesel engine is a success story. In 1893, Rudolf Diesel at Maschinenfabrik Augsburg developed a test engine that worked with the self-ignition combustion process, later to be known as the diesel process. It was followed by the first series-production model in 1897. Ever since, the diesel has established itself as a drive system in many industries worldwide. It’s only when installed in cars that the going gets heavy. Mercedes-Benz was the first automaker that, 80 years ago, ventured to install a self-igniter or compression ignition engine in a series-produced car. The 260 D from 1936 consumed a third less fuel than a comparable gasoline engine car. However, due to inferior performance and running properties, the diesel engine failed to gain acceptance among motorists. Right up to the 1980s, diesel engines were seen as the embodiment of efficiency, longevity, and reliability, at the same time, however, as sluggish and definitely not for motorists of a sporting nature. But the tide turned with the advent of turbo-charging and direct injection, which gave the fuel-efficient diesel engine the necessary dynamic properties. This was to spark the market success diesel cars now enjoy.

Indispensable in working toward climate protection goals

Diesel engines have never lost their fuel-efficiency edge over their gasoline cousins. The reason for this is the much higher pressure developed during the combustion process, which makes diesels up to 25 percent more fuel-efficient. Fuel consumption is directly coupled to carbon dioxide (CO2) emissions, because CO2 is inevitably generated when fuels containing carbon (such as diesel and gasoline) are burnt. When carbon burns, each of its atoms combines with two oxygen atoms to form CO2. This means that one liter of diesel releases 2.64 kg of CO2. So the 25 percent reduction in the diesel engine’s fuel consumption is equivalent to 15 percent fewer CO2 emissions, the difference between the two percentages being due to diesel’s higher carbon content. This significant reduction in CO2 emissions makes diesel cars indispensable on the road to achieving Europe’s climate protection goals.

Emission-control inside the engine

Yet where there’s light, there’s also darkness. The biggest challenge posed by the diesel engine is its relatively high share of non-CO2 emissions. Alongside particles, these are mainly nitrogen oxides (NOx). Like the lower fuel consumption, this is a result of the high pressure buildup in diesel engine cylinders during combustion. By fine-tuning the engine interior, this pollution can be pruned; although, this gives rise to a typical conflict of goals. Steps taken to lower CO2 often lead to higher NOx, while optimization of the combustion process with a view to reducing NOx would entail higher COemissions. There is, moreover, a close interdependence of NOx and particle formation. For if combustion is adjusted to reduce NOx, there is a corresponding rise in particle emissions, and vice versa. Any engine- related measures for curbing pollution always mean finding a compromise between these development goals. One established method adopted nowadays for optimizing fuel combustion in terms of controlling NOx emissions is exhaust-gas recirculation (EGR). Here, the exhaust gas is channeled away from the exhaust system, cooled, and then rerouted into the cylinders. Even though the exhaust gas fills the combustion chamber, due to its lower oxygen content it does not participate in the combustion process in the cylinder. As a result, combustion is generally “dampened,” and the peak temperature in the combustion chamber is lower. This results in a sharp reduction in NOx: a temperature reduction by 100°C in the combustion chamber is equivalent to a 50-percent reduction in NOx emissions. However, precise metering of the amount of waste gas in the cylinders is essential under these conditions, in order to avoid a sudden surge in soot emissions. To this end, pneumatic or electric return valves provide for high-precision dynamic control of the recirculated waste-gas volume under all engine operating conditions. Various types of exhaust-gas recirculation are used on today’s car diesel engines: high-pressure EGR, low-pressure EGR, and a mixture of the two. On the high-pressure variety, the exhaust gas is diverted from in front of the turbocharger turbine and routed to the intake side behind the compressor. This high-pressure version operates at the level of pressure developed by the turbocharger. On low-pressure EGR, the waste gas is diverted at a position behind the turbocharger turbine and fed back into the system in front of the compressor; the pressure level is more or less equal to the ambient pressure. The necessary pressure gradient is generated by exhaust flaps.  

Exhaust-gas aftertreatment removes harmful substances

Exhaust-gas recirculation offers the potential to reduce NOx emissions by around 40 percent. This is, however, insufficient for compliance with current statutory standards. So the exhaust-gas strategies adopted on modern diesel vehicles combine EGR with several waste-gas aftertreatment systems that help clean the gases outside the engine. The first stage outside the engine in the exhaust system is the catalytic converter where hydrocarbons, carbon monoxides, and particles react with each other and are neutralized. The downstream diesel particle filter removes residual soot from the exhaust gas. Thanks to this filter, the waste gas emitted by modern diesel engines is virtually sootless. To fight NOx emissions, auto manufacturers are using two different systems: the NOx storage catalyst and the SCR catalytic converter. The former stores the nitrogen oxides extracted from the exhaust gas until it has reached the limit of its intake capacity. Then, in the regeneration process, the stored NOx is converted into harmless components and the converter is prepared for the next NOx storage cycle. The NOx storage catalyst reaches an efficiency of about 80 percent. In the SCR catalytic converter (SCR = selective catalytic reduction), NOx is continuously decomposed by the nontoxic and odorless reducing agent “AdBlue,” which is sprayed as required into the exhaust flow upstream of the SCR catalytic converter. In the converter, AdBlue is decomposed to form ammonia which, in turn, reacts with the nitrogen oxides (NOx). Once the engine and the exhaust system have reached their operating temperatures, the SCR converter eliminates up to 90 percent of the exhaust’s NOx. With their high efficiency level, today’s exhaust-gas technologies possess the potential for solving emission problems and making the diesel engine clean by today’s standards.

The technology bridge to “zero emissions”

Exactly 80 years after its invention, Rudolf Diesel’s engine merits its place in the car more than ever. Innovative systems for emission reduction, inside and outside the engine, together with new testing methods, are making diesel engine emission issues a thing of the past. What remains is the diesel engine’s efficiency and CO2 benefits. And it’s these that make it indispensable in achieving the ambitious climate-protection and emission-reduction goals. As a bridge technology on the road to “zero emissions” through electric cars and regenerative energies, the diesel engine is a future technology of vast significance to the mobility of tomorrow.

Preventing losses: KSPG on the “Road to 95”

Starting from 2020, a limit of 95g CO2/km for new cars will come into force in the EU. Together with their suppliers, auto manufactures are working full speed at lowering their CO2 emissions to the permissible limit. Early on, KSPG bundled its related R&D efforts into a “Road to 95” package. The outcome: new strategies and innovative diesel and gasoline engine components for less CO2. These include fine-tuned cylinder assemblies complete with bearings in the basic engine. With new materials and coatings, KSPG has again greatly reduced friction within the engine. Electrically controlled exhaust-gas recirculation systems, air path valves and flaps plus exhaust flaps from KSPG improve engine combustion and emissions. On-demand ancillary components such as oil, coolant, and vacuum pumps prevent energy losses while the engine is running. The trend toward downsizing for increased efficiency through reduced cylinder displacement and the use of turbochargers is promoted by KSPG’s electrically actuated divert-air, waste gate, and pressure-control valves on exhaust-gas turbochargers. KSPG has also come up with an electric compressor that allows the charge pressure on charged engines to be spontaneously boosted. In this manner, the weak start-up characteristic on turbocharged engines, the “turbo lag,” can be overcome. Systems for “dethrottling” the charge cycles and throttle valves, reduce the losses during gas exchange within the cylinders. Plain bearings for transmissions and electric oil pumps are ways in which KSPG is fine-tuning the workings of these assemblies. Specifically with car diesel engines in mind, KSPG has developed pistons made not from aluminum but from steel. The latter lowers engine friction while allowing engine designers greater latitude.In their aggregate, these effects add up to more than three percent less CO2. The world’s first steel piston and recipient of the Steel Innovation prize went into series production at KSPG in 2015 and is being installed in a volume-produced car diesel engine. Likewise adapted to the specific operating environment of car diesels are the new electrically controlled bypass flaps for exhaust-gas recirculation modules. Normally, these flaps are vacuum actuated. Yet this vacuum has to be created which, in turn, produces CO2. KSPG’s proprietary electric actuator requires the energy, in fact, only at the moment of actuation.  


Richard Backhaus, automobile and motoring journalist


Diesel engines have been getting a lot of bad press lately. Will we really still need this form of automotive propulsion in future?

The diesel engine doesn’t deserve to be denounced as an environmental polluter. Today’s diesels combine low emissions with a low CO2 discharge.This is true both in theory and, much more importantly, in everyday road traffic, where diesel engines are preferred in big and hence heavy vehicles, which naturally consume more fuel than small cars and have an annual mileage that is significantly above average. Both these factors highlight the CO2 advantage of the diesel engine very clearly. Hence, the diesel engine is not a problem but part of the solution to the environmental challenges confronting personal mobility over the coming years. For, as is generally agreed, it will take sometime before electric cars actually gain broad market acceptance.