Car Design Q&A

By Eric Peters, Automotive Columnist

Q: Why haven’t any of the automakers built a diesel hybrid?

Diesel engines are more fuel-efficient than gas engines — by 20 percent or more, all else being equal. Wouldn’t it make more sense to use them in a hybrid vehicle rather than a gas engine?

The problem is that a diesel engine — like an electric motor — is optimized for low-RPM torque production. That’s great for a truck (or even a non-hybrid car) but not so good for a hybrid — which plays on the relative strengths of the electric motor (lots of low-speed torque for getting the car moving and for stop-and-go type driving) and the gas engine for extra power when passing and operating at higher speeds, as on the highway.

Diesel engines also produce different emissions as a byproduct of combustion than do gasoline-burning engines. Instead of catalytic converters, a diesel hybrid would probably need particulate trap, urea injection and so on — all of which would result in a diesel hybrid that’s more expensive (by about 20 percent) than a gasoline-electric hybrid.

Bu they are working on it. In Europe, where gas costs $8 or more per gallon, several automakers are looking at diesel hybrids.

Q: How come modern economy cars don’t get much better gas mileage than the economy cars of 20 years ago? Hasn’t technology improved?

Absolutely. But even though modern engines — especially those equipped with technologies like direct fuel injection, variable cam timing, cylinder deactivation, etc. — are more efficient than the engines of the past, they are also larger and more powerful.

And the reason for that is the higher curb weight of the typical 2010 model year economy car. On average, a ’10 economy compact weighs about 500 pounds more than its 1980s-era equivalent. An there are two reasons for that: One, federal crashworthiness requirements are much stricter today than they were decades ago. To meet these requirements, automakers have bulked up all their new cars to make them safer to be in if there’s a crash. Also, all new cars now come with things like dual air bags (at minimum), which adds more weight to the car. Finally, most people want things like air conditioning and power windows — even in “economy” cars. These creature comforts also beef up the car’s curb weight.

In turn, this has required larger, more powerful engines. A modern economy car’s engine is around 1.6-2.4 liters in displacement and produces 110-140 hp. Back in the Reagan Era, many economy cars had engines that were smaller — and made less than 100 hp.

Q: Whatever happened to “real” chrome bumpers?

They’re still there; they’re just no longer visible — or chrome-plated. Instead, they’re hidden behind flexible plastic “fascias” — the auto industry term for the body-colored front (and rear) panels that give modern cars their “bumperless” look.

This change happened for two reasons: One, the “bumperless ” look looks good to most people. It gives the car a seamless, one-piece appearance. It’s also more aerodynamic, which improves the car’s drag coefficient at highway speeds, which improves fuel economy and also makes the car quieter inside.

Two, chrome plating is expensive as well as environmentally toxic — so the automakers had two good reasons to shelve exposed chrome bumpers in favor of those body-colored fascias (and hidden bumpers that don’t have to be chrome plated).

The downside is that plastic fascias are easy to damage and very expensive to repair — which is one reason why insurance costs for modern cars are so high.

Q: Why do many front-wheel-drive (FWD) cars still have a driveshaft “hump” on the floorboard?

One of the design advantages of the FWD layout is the entire drivetrain — engine, transmission and axle — are packaged together up front, eliminating the need for the hump in the floorboard (in a rear-drive car) that would otherwise need to be there for the driveshaft, which in a rear-drivecar runs from the engine up front to the drive axles out back. The hump divides the interior space of the cabin straight down the middle, turning a potential six-passenger car into a five-seater. Early FWD cars like the 1960s-era Oldsmobile Toronado boasted huge interiors with considerably more room for passengers than an otherwise similar rear drive car.

But modern FWD cars also sit very low to the ground — and designers have found a new use for the “driveshaft” tunnel as a place to tuck the exhaust pipe and other components so they don’t scrap the ground. The downside is the “flat floorplan” (and more usable interior space) you’d normally get as a benefit of the FWD layout has been lost.

Q: It says “4WD” on the bumper, but there’s no Low range. What gives?

Technically, both traditional truck-style four-wheel-drive systems with a two-speed transfer case (and Low range gearing) and the increasingly popular all-wheel-drive systems being offered in many cars, wagons and light-duty SUVs are both “4WD” — since both systems do, in fact, drive all four wheels – – at least part of the time.

But the marketing of all-wheel-drive as “4WD” is a bit deceptive, since that term (4WD) has traditionally been used to refer to a heavy-duty, truck-based system with a two-speed transfer case and Low-range gearing designed for dealing with deep snow, mud and so on.

Reason? “4WD” sounds more masculine than “AWD” — which is why several automakers describe their lighter-duty AWD systems as “4WD.” It’s technically accurate — but also a bit deceptive. Before you buy, be sure you know what you’re buying and that it meets your needs — no matter what it says on the bumper.

Q: Is it just me, or have doors been getting higher (or seats getting lower?)

Both. If you’re old enough to remember the cars of the ’80s and before, you also remember that you used to be able to comfortably rest your left arm on the top of the door, with the window rolled down, on warm sunny days. That is increasingly hard to do on most new cars.

Reason? In order to improve occupant protection in a side-impact crash, doors have been “built up” so that more reinforced steel (and less glass) is between you and that SUV that just ran the red light. There are also design considerations having to do with the placement of side-impact air bags. You may not be able to rest your left arm on the sill as you drive – but you’ll be safer if someone T-bones you at an intersection.


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