Diesel Cars in the US

I regularly hear and read claims that there is some industry conspiracy against bringing diesel cars to the US, or that Americans won't buy diesels because of the bad taste left behind by the terrible diesel products of the late 70's.

Both claims are, to use an old fashioned word, bunk. Here is a short explanation of why I think diesels are not being introduced yet.

The most significant engineering problem with using diesel engines in light duty vehicles, at this point, is the EPA and the California Air Resource Board (CARB). The EPA has mandated that light duty vehicles meet very stringent emissions standards, called Tier II. Because of their inherently high NOX emissions, diesels can not (cheaply) meet these emissions standards. Diesels can be made to meet these standards with a few tricks. CARB's rules are even more stringent than EPA's--diesel passenger cars have not been certified for sale in California since 2003.

First, clean diesel fuel is necessary, with low sulfur levels, so that catalysts can be used to change the NOX into N2 and O2. Without low-sulfur diesel, the catalysts get "poisoned" by the sulfur in the fuel, and can not do their job. Cleaner diesel fuel should start showing up at the pumps in 2006.

An alternative to using a precious metals based catalyst for NOX controls is to use urea injection, where the chemical is injected in the hot exhaust stream. GM is proposing to use this system for its pickup trucks, but EPA has not approved it yet, as the system would require the driver to keep a urea tank filled.

The other major diesel emissions issue is particulate emissions, or soot. The current solution is to capture the particulates in a particulate trap oxidation catalyst at lower temperatures, and burn the particulate at high temperatures.

As to the noisiness and hard-to-start legend of diesel passenger cars, the myth would be dispelled the moment that modern diesel engine cars from Europe would be demonstrated here. American's aren't stupid, and we don't have a very long memory (not necessarily a good thing). The real consumer objection to diesel performance is that they are low horsepower, high torque engines. Diesel engines don't like to spin fast (peak power might be at 4000RPM, dropping dramatically at higher speeds). In order to get performance closer to a gasoline engine, diesel engines need to be turbocharged (like the VW TDI), and also may need an intercooler.

Diesel engines, because of their higher internal pressures, and high torque output, need to have much stronger internals then a corresponding gasoline engine. A diesel engine may have a compression ratio of 20:1, compared to a gasoline compression ratio of 10:1. A diesel engine will tend to be heavier than a comparable gas engine.

The problem with diesel engines for light duty passenger vehicles boils down to basically one thing: cost. To have a diesel engine that meets EPA and CARB requirements, you need to have a complicated and expensive dual catalyst or urea injection + catalyst system. You need to have a turbocharger and intercooler. You need a much stronger engine block, crank, pistons, connecting rods, valves, and head. You may wind up with an engine that costs several thousand dollars more than a comparable gasoline engine. At which point, the added cost of the powertrain outweighs the short term fuel savings (like HEVs), and the average consumer has no great incentive to buy a diesel powered car.

If the "value equation" isn't right, there is no way that automakers are going to invest heavily in passenger car diesels.