Diesels not only have unique starting characteristics but also unique running characteristics. The "low" horsepower ratings of Diesels scare away much of the North American public (pity), yet your first driving impression is actually how much power you actually feel. This is because the torque value is pretty close to that of an equivalent displacement gas engine, but the torque comes on strong at a much lower RPM and stays strong right to the upper rev limit. It is actually torque (rotational force) that moves a car anyway. The net result is that a "mere" 77 HP can make a 3600 lb car very drivable- if it was only a 77 HP gas engine in such a heavy car, it would not be drivable at all.

The difference comes forth as efficiency as opposed to raw performance. The engine rises to peak efficiency very quickly and stays near that peak at all times. Occasionally, demand for power will exceed the efficiency limit- such as climbing a steep hill, and the car will typically lose a bit of speed. But the reward is 30 to 50% increase in fuel economy when compared to similar displacement gas engine moving similar weight.

The latest car advertizing seems to emphasize horsepower again- we're seeing near 300 HP gas engines in ordinary mid size cars and mini vans. These cars have an abundance of HP and torque on tap to exceed the occasional peak demand, but in normal driving, only a fraction of the power is being put to use- which is very inefficient, which shows up as bad fuel economy. Modern technology in the engine compartment may provide better fuel economy than say 5 years ago, but still not as good as Diesels. Still, comparison between gas and Diesel engines is not exactly apples to apples. The "performance" aspect of a Diesel lies in a hard to define, but easily felt experience of drivability.

My favourite experience of Diesel drivability came from a '89 VW Diesel I once had. It only had 52 HP. On a few occasions, I had 3 male co-workers on board out on our lunch break. There was a rather steep incline from out of the parking garage, and I always had great fun with about 800 pounds of human cargo including myself, and the car would actually IDLE up the incline in 1st gear without stalling! Try that with a gas engine of ANY HP- it simply wouldn't work. Then at the top of the incline I would simply press on the accelerator without slipping the clutch at all, and merge effortlessly into traffic.

The reasons for this aspect of Diesel performance is largely due to the engine's compression. Remember when high compression gas engines ruled the streets- maybe about 12:1 running on premium gas. Well, Diesels are 20:1, and the higher the compression of air within the engine, the better the efficiency. Keep in mind that this extreme high compression alone actually contributes energy back into the power stroke, so that the combustion of the fuel actually has less work to do. When you add a turbocharger, you literally recover free power from the engine's exhaust. The turbo is driven by the engine's wasted energy- the scaping exhaust gases, and returns that energy to the engine's compression stroke. As a result, the pumping action of the pistons does not have to contribute all the compression, as the turbo has pre-compressed the air somewhat. But unlike with a turbocharged gas engine, the increase turbo-Diesel airflow does not demand more fuel. Fuel economy of normal Diesels and turbo-Diesels is pretty much the same, because the Diesel's charged intake only consists of air, instead of an air-fuel mixture. Net result is that the waste- exhaust energy is put back to the drive wheels without the need for more fuel.

Gas engines require a constant air-fuel mixture of 18:1 in order to run right, thus if you turbocharge the air flow, more fuel must be added to the air to maintain this ratio. Diesels on the other hand run with air - fuel ratio's that range from an extremely thin 200:1 at idle speed to 15:1 at full speed. Hence, in normal driving, the air - fuel ratio is almost always better than that of a gas engine's constant requirement of 18:1, and having the turbo does not increase fuel demand, rather it "helps" by adding free compresion of the air into the engine. This explains how a Diesel's fuel economy is just as good in city driving as it is with highway. When sitting at a red light, your Diesel is consuming virtually no fuel- just enough to keep it ticking over against the engine's compression.

Finally, when it comes to fuels, Diesels are not near as fussy as gas engines. Anything that ignites well with heat will do- and that includes very combustable french fry oil- a leading cause of house fires- right? Gas engines on the other hand require fuels that ignite with spark- true of gasoline, alcohol, or propane, but not true of the oily fuels such as Diesel, Kerosene, or vegitable oil.

Occasional "man of science"

Dave

1976 White 300D W115 "Pearl"