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A Diesel in Your VIN?

How diesel engines work

It's the 8th digit. No, we don't mean some new movie, starring actor Vin Diesel, or the name of a new line of designer clothing. We're talking about cars, and the VIN is an acronym for Vehicle Identification Number. It's that small metal tag, usually mounted on the driver's side-front corner of the dashboard. When you view the VIN through the windshield from outside of your car, you can see it's inscribed with a cryptic series of letters and numbers. And the 8th digit in the series refers to what kind of engine is under the hood.

But you really don't need to know about the 8th digit to recognize a diesel-powered vehicle, do you? Everybody knows they're noisy, smelly, smoky and slow. That's certainly true about diesel-powered cars of the past, but what's changing all of that?

Diesel History

Looking at the background of the diesel engine, it has been around for as long as gasoline-powered, internal combustion engines. It didn't see much use until it proved itself as an industrial workhorse that's durable, fuel-efficient, low-maintenance and relatively clean running (at least compared to the coal-fueled steam engine). Of course, diesel engines continue to be a mainstay in the industrial, heavy-equipment, and trucking sectors, but that's not our focus here.

They were used in passenger cars sporadically throughout the early days of the automobile, but didn't catch on until the contrived "fuel crisis" of the Seventies reared its ugly head. At that point, it was the European auto manufacturers that had a head start on putting the diesel-powered rubber to the road, exploiting the virtues of the technology, while (at least for the time) minimizing the vices. A large enough segment of the U.S. population agreed then, and even today, that the turbo-diesel powered sedans manufactured during that era, most notably from Mercedes-Benz, are viewed with something akin to reverence. Many are still in daily service, with odometer readings at or above 300,000 miles! In addition, the Volkswagen Rabbit diesel set the benchmark for fuel economy during this period (and also for leisurely acceleration).

This success did not go unnoticed by the U.S. manufacturers, who, by the beginning of the Eighties, had produced what they believed were cars that could directly compete with these European products. All of which begs the question for U.S. customers unfamiliar with diesel technology: how does a diesel differ from the gasoline engine that we are so accustomed to? Moreover, what are the benefits of going the diesel route, and how has this engine been improved in recent years?

Diesel Explained

The diesel engine differs not only in the fuel used, but also in the method used to ignite it in the combustion chamber. For comparison, a gasoline engine admits the fuel/air mixture into the combustion chamber by opening a valve, allowing the downward movement of the piston in its cylinder to draw the mixture past the valve, then closing it. Now, with the combustion chamber sealed (valves closed), the piston moves upward, compressing the air/fuel mixture in the process. A spark from the spark plug ignites the mixture, driving the piston downward, which turns the crankshaft. Exhaust by-products are expelled past an opened exhaust valve, as the piston (moved by inertia from the combustion process described) approaches the top of its travel on the next cycle. The process is then repeated. The relatively high "volatility"—propensity to burn—of gasoline is perfect for this process.

The diesel engine uses a similar valve/piston arrangement, with the same distinct four cycles that we've just described. The difference here is that, when the intake valve opens, only air enters into the combustion chamber. Then, due to a diesel's much smaller combustion chamber design, when the piston reaches the top of its travel, this compression process has super-heated the air. It is at this point that the fuel is injected under very high pressure into the combustion chamber, where it is ignited by the super-heated air, driving the piston downward. The exhaust cycle is the same as in the gasoline engine.

Diesel fuel, similar to kerosene or the fuel used in jet airliners—with its much lower volatility, yet high energy potential—is the perfect fuel for this process. Without getting heavily into physics here, it can be said that the horsepower and torque characteristics of these two designs are distinctly different. Horsepower essentially is a way to measure expended energy, while torque is a way to measure twisting force exerted (as on a driveshaft, for instance). Gasoline engines produce higher amounts of horsepower per cylinder volume than do diesels. Diesels, on the other hand, generate more torque than gasoline engines. Torque, in a diesel engine, can be produced prodigiously at a relatively slow engine speed. Therefore, diesel engine can do a given amount of work using less fuel than a gasoline engine.

Diesel Evolved

Typically, the diesel's lack of horsepower has been compensated for somewhat by the use of induction turbochargers, which force-feed air into the cylinders (instead of being drawn in by downward piston movement), making it behave like an engine of larger cylinder volume. All of this has been proven possible along with a high measure of reliability, and low maintenance costs, due in part to the diesel engine's generally beefier construction. That extra beef, while necessary to keep the engine durable due to the higher forces at work, does add weight to a vehicle. The higher internal pressures often mean more noise, vibration, and harshness (NVH). And then there are increased exhaust emissions, especially particles. (Most people would refer to this as smoke or soot.) Auto manufacturers have addressed these issues by applying the same technology used to make gasoline-powered vehicles more driveable, powerful, fuel efficient and emissions-friendly. Cars such as the VW Beetle with its 1.9-liter turbo-diesel engine are selling well, and are much friendlier to the environment—besides having more power and exhibiting increased fuel efficiency, compared to diesels of 20 years ago. So don't be surprised if you start seeing more diesels in a VIN.