Power Trains

Take a close look at a steam locomotive. Notice the mechanical simplicity: pistons directly driving wheels. No clutch, no complex gearing and shift mechanism. Yet these mechanical marvels of their day delivered as much as 4000 horsepower and pulled thousands of tons of manifest.

In contrast, conventional automobiles of today require a some kind of clutch and multiple sets of gears between the engine shaft and the wheels. The mechanism is all but exposed to drivers of manual transmission vehicles, such as high performance cars and large trucks.

Why is it that small cars need a clutch and transmission, and large high powered steam engines didn't?

The answer is simple: stall torque. Stall torque is the torque supplied by a power plant at zero angular velocity or RPM. In order for a car, truck, or train to get started, it needs a push. The bigger the push, the faster it will accelerate. The problem is further exacerabated by static friction and gravity on a hill or even a small grade. Anyone who has run out of gas and tried to push their car has had first hand experience.

The problem is that internal combustion engines, like gasoline engines, diesel engines, even turboshaft engines do not produce any stall torque. They can't, because they stall at low speeds. Internal combustion engines idle at anywhere from a few hundred RPM to several thousand. A car has to start from zero, so there is a significant speed mismatch. The common solution is to introduce a clutch or torque converter to apply enough torque to the wheels without stalling the engine. If you can remember learning to drive a manual transmission car, you know how tricky the transition from a stop can be.

Another problem is that internal combustion engines have a limited power band. They can only produce useful power and torque for a limited range of RPM. Unfortunately, the range of driving speeds we are accustomed to exceeds the power band of most commercial engines. To deal with this problem, a transmission is placed between the clutch and drive shaft. The transmission effectively widens the power band by shifting smaller and larger gears at different speeds. This keeps the engine RPM within its power band. It's not perfect, but works and it has been used for over 100 years.

Obviously the clutch and transmission adds complexity, cost, and weight to a car, but it also reduces efficiency. Did you know it only takes about 70 horsepower to accelerate a 2000 lb car from zero to 60 mph in 6 seconds? To achieve that same performance, you would need a gasoline engine that produces almost three times that power. Internal combustion engines operate at maximum power at exactly one speed. Ideally, the engine should be operated at a fixed RPM or at least a very narrow range to get the most power and efficiency. This can be accomplished with a continuously variable transmission or CVT. Mechanical CVTs have been built for a long time, but they are generally inefficient for a number of practical reasons.

Steam locomotives don't have any of these problems because they produce lots of torque all the way down to zero RPM. It's easy to see the torque in action as a steam is allowed to build in the pistons and even the longest of trains slowly crawl forward and gain speed. Of course steam engines also have their limits, many steam trains could not travel faster than 60 mph, but that's still much better than first and second gear in most cars! Steam locomotives were replaced by diesel-electrics in the late 1940's and 1950's in North America and Europe. Stream engines required a lot of maintainence and expertise to operate. The lack of a suitable condenser also limited range because the water in the boiler would eventually be used up.

So how to do modern diesel locomotives solve the torque problem? They also use a relatively simple system, but it's a hybrid electro-mechanical drive. Instead of driving the wheels through a direct mechanical linkage, diesel-electric locomotives (and most other electric trains, such as subways) use electric motors to drive the wheels. The electric power for the motors comes from a generator driven directly by the engine (or power grid in the case of a subway).

This seems counter intuitive. Why take mechanical power, convert it to electrical power using a generator, and then convert it back to mechanical power using a motor? The answer has to do with the nature of electric motors. Believe it or not, electric motors (and generators) can be very efficient, as high as 95% or more. In some cases, this is more efficient than mechanical transmissions, all those sliding helical gears generate heat and waste power. More importantly, electric motors can produce high stall torque, much like a steam engine. Due to conservation of energy, the torque produced by a motor is proportional to the electric current in its coils -- the greater the current, the greater the torque. There is a limit on the torque for a given size, this is due primarily to magnetic saturation of the steel typically used in motor cores.

Most traction motors used in diesel electric locomotives and subways are integrated with the trucks. Some are even part of the axles. Hybrid drives are used in many types of vehicles, including large construction equipment such as earth movers, large sea vessels, aircraft carriers, even nuclear submarines. Since the torque (and speed) of electric motors can be independently controlled electronically, it makes for a smooth drive. Operating an electric drive is trivial: just turn a small knob to add torque. The greater the torque, the greater the acceleration. To get the same effect in typical car, you have to shift through several gears and adjust the throttle. I had the good fortune of touring an operating diesel electric locomotive in 2004. It's surprisingly simple!

So why aren't electromechanical drives used in cars and trucks? It's largely a problem of economy of scale. The old clutch-transmission system has been produced for more than a 100 years, so it's cheap. Toyota's Hybrid Synergy Drive uses both an ingenious transmission and an electric motor/generator to achieve smooth drive and high efficiency. It's a step in the direction of pure motor drive, but today it costs more than a conventional transmission.

Theoretically, it's possible to build a one piece motor generator that replaces the clutch and transmission in a conventional automobile powertrain. The guts are really quite simple and require almost no maintainence. Motors and generators are no more than steel, wire, and one moving part. The control system is less complex than todays electronic automatic transmission controls. After 100 years of gears, we have become gear heads. It's hard to change to something else, even though it's simpler, more efficient, lower maintainence, and smoother. As gas prices rise, the economics will catch up, but who knows for certain when that will be.