One reader castigated me for not paying enough attention to Thielert diesels. Another inquired whether the just-certified Diamond TwinStar would have superior climb performance because of the great torque of its Thielert diesel engines. A critical mass of Thielert interest has evidently been reached.
Thielert GmbH is located at the small town of Lichtenstein in the former East Germany (not to be confused with the principality of Liechtenstein, wedged almost invisibly between Switzerland and Austria). The company produces, among other things, liquid-cooled geared turbo-diesel aircraft engines based on Mercedes automotive designs. A 1.7-liter, 135-hp four-cylinder is currently certified for retrofit to Cessna 172s and Piper Warriors originally equipped with the 160-hp Lycoming O-320 engine. The Austrian firm of Diamond offers its single-engine Diamond Star with one of the engines, and the upcoming TwinStar has two. More than a hundred Thielert-powered airplanes are now flying, and the company expects to certify a 4.0-liter, 310-hp V-8 this year.
It's unusual to offer an engine of lower power to replace one of higher. One wonders where the company gets the chutzpah to do it. The answer is that, at least in the 172 and the Warrior, performance is barely affected, gaining a little in some areas and losing a little in others. To understand how this can be so when the new engine is apparently much weaker than the old, we need to look at the characteristics of diesel and gasoline engines.
To start with, their combustion cycles are fundamentally different. The gasoline engine draws a fuel-air mixture into the cylinder, compresses it, and finally ignites it with a spark. The need to avoid spontaneous ignition (either pre-ignition or detonation) limits the compression ratio, while the need to ignite and burn the compressed mixture requires that the ratio of fuel to air remain within fairly narrow limits. An air throttle, linked to a fuel valve in injected engines, regulates power, while a mixture control allows fine adjustment of fuel flow in order to ensure a combustible, but not wasteful, fuel-air ratio.
The Thielert engines have no air throttle. Only engine speed regulates the amount of air they take in, while fuel flow controls their power output. Rather than a fuel-air mixture, only air is compressed by the rising piston. A very high compression ratio-18:1, more than double that of a typical unsupercharged aircraft gasoline engine-raises the temperature of the air in the cylinder (because the same amount of heat energy gets crammed into a much smaller volume of air) above the point at which hydrocarbon fuels ignite spontaneously. No timed spark is needed; fuel, sprayed into the cylinder under extremely high pressure as the piston reaches the top of its travel, burns regardless of the fuel-air ratio, which is always, by gasoline-engine standards, very lean. There is no harm, after all, in having excess air being present, just so long as there is enough oxygen to react with all of the hydrocarbons in the fuel. Jet engines, too, take in far more air than they use for combustion.
In their familiar roles as powerplants for generators, trucks and ships, diesel engines are typically designed to be slow-turning but to have a long stroke. The stroke, which is the distance the piston travels, is twice the crankshaft's "throw"; the throw, in turn, is the maximum length of the lever arm against which the piston exerts its pressure. Long throws ensure that diesels yield plentiful torque. Since horsepower is the product of torque and engine speed, a slow-turning engine with lots of torque can work as hard (that is, produce the same horsepower) as a faster-turning engine with lower torque. Torque is naturally determined not by crank geometry alone, however, but also by the pressure in the cylinder during the power stroke and by the area of the piston (which is as much as to say the "bore" of the engine).
Although the 135-hp Thielert Centurion 1.7's weight, around 300 pounds, is similar to that of the 160-hp Lycoming O-320, its displacement is less than a third of the Lycoming's. It achieves its maximum power at 2300 prop rpm-3900 crank rpm-versus 2700 for the Lycoming.
