Before the Diesel engine became cleaner, i.e., emitted less soot particles and NOX, it was first made even more economical. The direct-injection which was already available for utility vehicles in the 1960s, was, first of all, installed by Ford in the harshly running Transit, somewhat later, it was presented by Audi in a refined version for normal motor cars. What the customers also noticed, was the unusual willingness of this type of engine to perform and even moreso, the early delivery of torque.
Now the consumers were given the choice, a clean petrol engine or an economical Diesel. The petrol engine, compared to what it was then, had indeed become more economical, it could not however, hold it's own against the direct-injection Diesel. On the other hand, the Diesel could not allay the suspicion that it was blowing cancer-causing substances into the environment. Around the end of the century, the petrol engine was also given the direct-injection and the Diesel got an even more difficult to manage exhaust gas treatment.
Just how difficult it was to master these two technologies, is shown by two, for this field, unusual flops. VW propagated, with a great deal of effort, the FSI-charged (Fuel Stratified Injection) petrol engines. They were however, only effective when they were driven very gently, no longer suitable for use on the motorways. Later, at VW, this system was again homogenized. The manufacturers were promising open exhaust systems, which would substantially reduce the particle emission. Subsidies were granted by the legislation. The only problem was, the installed systems proved ineffective.
Towards the end of our observation period, what we have, is the more or less complicated (mostly turbo) charged Diesel motor car engine, mostly with post-injection, regenerable particle filter and any number of detoxified exhaust gas components. This begins with the two-stage, cooled exhaust gas recirculation, going on to the multi-injecting common-rail system on the low-pressure side. After holding on to the pump-jet process for a long time, VW has now also gone over to the common-rail system. An interesting point is that Daimler is now challenging VW for the position of being the most economic Diesel engine manufacturer and BMW is also not far behind them.
As far as charging is concerned, the petrol engine was held back in the beginning, then they appeared to resort to the compressor, now the situation is such (similar to the Diesel), that we've come to the point where the combination of the two is no longer transparent. Indeed, the engines of the more reasonably priced models have not been considered for these combinations. Which is also why they seldom fall into the category of fuel savers. The old consumption difference between petrol- and Diesel engines has again been stabilized. One can however, not believe the statements about fuel consumption which the manufacturers advertise. At best, one can only get somewhere near to them.
To close off with, it would be nice if we could pick out one type of piston-engine to stand as a monument for the past, at least 110 years. Certainly, one of them is inconspicuous, it does it's job today almost unnoticed and probably will do for many years to come. What we're talking about, is the four-cylinder in-line engine. It was not created at exactly the same time as the first combustion engines, indeed, a good 10 years later it was there, not as a unit but as a 2x2. It led the Daimler-motor-works to undreamed of racing successes, without it, they would probably never have got to where they are now. From that time onwards, the propulsion of vehicles without it, was unthinkable.
Admittedly, it can't compare with the smooth running and low center of gravity that the four cylinder boxer has. Therefore, it can be mounted straight or transverse in the engine compartment. Should Ferdinand Porsche not have been so shot on an air-cooled rear-engine, he would have chosen an in-line unit. The second most frequent car in the history of the automobile had one. A replacement is, still today not in sight. On the contrary, even those commited advocates of the six-cylinder, like BMW, are recently installing them more and more, they are even being used in cars with the most modern technology, like the Toyota Prius and the Opel Ampera (Chevrolet Volt).