Why Diesel Engines Produce So Much Clatter

Question: What is the reason that diesel engines produce a clatter not heard in gasoline engines? --P.S.

Answer: The clatter results from the combustion of diesel fuel inside the engine. In a diesel, the fuel is ignited by high pressure and temperature inside the cylinder, rather than by a spark plug.

The clatter is the result of fuel not burning as evenly as in a gasoline engine, creating a knock. Unlike knock in a gasoline engine, the knock in the diesel is not generally a threat to the engine. It is simply offensive to people.

Since the diesel engine does not have a spark plug and can ignite only from the compressive heat inside the cylinder, the fuel must have very good ignition qualities. But good ignition quality in any fuel is inconsistent with good anti-knock capability. Since diesel fuel requires a very good ignition capability, it has a rather poor anti-knock capability.

Q: A long time ago, you published a list of cars that were the most frequently stolen. I was wondering if you have any more recent information. I am replacing a car that was stolen and don’t want it to happen again.--T.M.

A: The type of car people own certainly affects their vulnerability to theft, but a lot of other factors come into play. If you park your car overnight on the street or if it is parked during the day in a public area, you are far more likely to be the target of a theft. The neighborhood and city in which you live obviously make a big difference.

Then there are the General Motors cars. In 1989, GM cars held nine out of the top 10 positions in the list of most frequently stolen cars, according to a recent list compiled by CCC Information Services, an insurance valuation company in Chicago.

Late-model Chevrolet Camaros are so popular with thieves that they are the top four cars stolen, followed by two Firebird models and two Oldsmobile Cutlass models. The Hyundai Excel also shows up on the top 10 list.

By comparison, the Ford Mustang was the 43rd most stolen car in 1989, the only non-GM domestic car to make the list.

GM cars are obviously popular among thieves because of the demand for parts. Many GM parts are interchangeable between models, which adds to the demand for stolen parts.

These cars are also easy to steal. I have a videotape that details theft techniques, and GM door and ignition locks can be defeated by an experienced thief in seconds. The videotape was produced by a repo man, somebody who is hired by lending institutions to repossess cars.

GM is doing something about the problem. The auto maker has introduced a new anti-theft system, called the PASS-Key, that shuts down the fuel system if somebody tampers with the ignition system. It is now found on the 1989 Camaro.

I want to compliment the several readers who faulted a recent answer of mine to P.S.W., who complained that his 1988 Toyota Camry is noisier than his 1986 Buick Somerset. I told him that the reason was possibly that his old Buick was heavier and perhaps had a V-8 engine. Heavier cars absorb sound better--and V-8 engines generate less noise--than four-cylinder cars.

I based my answer on some advice from automotive experts. But several readers who are obviously experts in their own right pointed out that the 1986 Buick Somerset did not come with a V-8 option.

Since then, I have also received a good deal of comment from current and former Camry motorists about the noise levels in their cars. Having recently test-driven a 1988 Camry, I would agree with P.S.W. that the car is noisy, not just compared to his Buick but to many other compact cars with four- or six-cylinder engines.

Vartabedian cannot answer mail personally but will respond in this column to automotive questions of general interest.