Engine displacement is no longer a yardstick…
The original Volkswagen Kombi was such a brilliant idea that other companies can be forgiven for copying it. Early Volkswagens were seriously underpowered, but they at least had four cylinders. One of the early German copies was the Atlas, produced by the now-defunct Gutbrod Company whose claim to fame lies in the fact that they were the first automotive brand to offer fuel injection in a petrol-engined car. This happened in 1953, about one year before Mercedes-Benz employed fuel injection in the 300SL. The Atlas had an air-cooled engine at the rear, just like the Kombi, but it was a 576 cm3 two-stroke twin developing only 21 kW at 3 500 r/min.
Sometime in the 50s I happened to be in a Cape Town showroom where a new Gutbrod Atlas was on display, and I overheard the conversation between a prospective buyer and a salesman.
Buyer: “Has this bus got six or eight cylinders?”
Salesman: “No sir, it has only two cylinders, but it’s very powerful because it’s a two-stroke.”
Buyer: (to his friend) “Koos, let’s get out of here. This youngster is lying to us.“
There’s a good chance that similar conversations will take place in the near future in showrooms all over the world, because the rush to downsize has infected just about every boardroom in the business.
Part of the problem, from a customer’s point of view, has been the fact that, historically, an engine’s displacement has been seen as a yardstick by which performance can be judged. This was actually a valid criterion in the days before WW1, when most engines operated with similar average pressures on their pistons, and revved to not much more than 2 500 r/min.
But by the end of WW1 the lessons learnt from designing aero engines had influenced engine design to such an extent the cubic capacity yardstick had to be linked to valve gear layout, compression ratio and maximum revs before any judgment of a particular design could be made. This follows from the fact that a one litre engine revving to 6 000 r/min sucks is as much air in one minute as a two-litre revving to only 3 000 r/min.
At present, many mass-produced engines in the 1,0 to 1,6-litre class have either been replaced or will soon be replaced by three-cylinder units. Thus urge to go smaller even applies to luxury brands like Mercedes-Benz, BMW and Audi. Fiat is producing an engine with two cylinders, and others are bound to follow.
What are the advantages of a triple compared to a four? Improved fuel consumption is the major advantage, because one piston with its rings, con-rods, bearings, valves, valve gear and reciprocating masses are gone. In most designs the consumption is expected to be improved by 15 to 20 per cent, but there is seldom a loss of performance because turbocharging is often employed to restore the output to four-cylinder levels.
Three-cylinder engines are not as smooth as fours, because the firing of the end cylinders at different times feeds a rocking motion into the cylinder block. This can be countered by employing an engine-speed balance shaft that creates an opposite-phase rocking motion. Liquid-filled engine mountings also help to civilize these engines and I look forward to driving some of the latest designs.
