Ditch Gasoline?!
[ April 14th, 2008 ] By: Mark Ozimek Posted in » Questions, Ramblings
So Charles and I were talking about the engines in rally cars the other day, specifically how the horsepower in the WRC is limited to 300hp. If you have read my article on engine power and torque, you’ll realize that this means they have the option to make amazing amounts of torque at lower rpm while staying under that 300hp limit. It is possible to design a gasoline engine to stay close to 300hp for a good part of the rpm band. Just size the turbo right so that there is a lot of boost down low and use a control system to taper off the boost in higher rpm to not exceed the power limits, coupled with a well-configured camshaft profile and such.
This type of power output curve very closely resembles that of an electric motor. The most torque is seen at or very close to 0 rpm, with the power output being pretty constant through the rpm range. This torque makes an electric motor great for starting off the line, or coming out of slow corners with lots of acceleration. There is another aspect of an electric motor that makes it far superior for the responsiveness that rally racing requires: the power response is instantaneous. With an internal combustion engine (abbreviated ICE), the throttle valve opens when you push the pedal down. This allows more air to flow into the engine, the ECU will see this through various monitoring methods and inject for fuel to keep the mixture close to what is required for the conditions (usually around 12:1 to 14:1). Then the exhaust gas flows through the turbine wheel, makes the turbine and compressor spin faster, increasing the pressure in the intake, causing even more air to enter the engine, creating more power. As you can see, there are quite a few steps involved here. The throttle response of a typical ICE is considered to be pretty fast, which is why they are used in cars, but in comparison, the electric motor is much must faster.
Simply press the accelerator pedal (not a throttle anymore!), the power control circuitry will allow more current to flow through the motor, and the motor creates more torque. No waiting for air to accelerate into the engine, no waiting for the ECU to compensate for this extra air with more fuel, no waiting for the exhaust to flow past the turbine to spool it up. It’s nearly instantaneous in comparison.
There are quite a few other advantages to electric motors aside from this. The thermal efficiency is often >80%, while a typical high performance ICE will be lucky to get 15% efficiency out of the gasoline it burns. The overall package size and weight of the engine itself is greatly in favor of the electric motor. Same with reliability: a gasoline engine has a lot of moving parts from the crankshaft up to the valvetrain, while an electric motor just has the core that spins.
The two of us sat there and discussed this topic for quite a while, and realized how amazing it would be. There is just one significant technical hurdle left to overcome: the batteries. Getting the range and power output needed for a rally car will weigh a lot. However, it is definitely something to consider as a possibility for the future of rally racing. What do you think about it? What other unforeseen problems do you think there would be? We would really like to hear what you think.
April 15th, 2008 at 11:36 am
What about the long term lifespan of the batteries? How frequently would they have to be replaced?
April 15th, 2008 at 4:18 pm
You sir, have touched on the issue Mark and I were stumbling on. The batteries just aren’t there yet, and we guess you’d have to change all the batteries out every service (because charging is too slow). Although you could probably reuse the batteries every other set of stages.
April 15th, 2008 at 6:25 pm
Most batteries are good for >500 cycles before the capacity decreases significantly if properly cared for. There are several factors in this though, such as charging voltage, operating temperature, discharge current, and a few others. The discharge current is really the sticky point here. Keep in mind that one horsepower is 745.7 watts. So to supply an electric motor in a rally car with power, a battery pack is going to have to be able to put out >150 kilowatts. Let’s say for the sake of argument that we design a battery pack to put out 250 volts. Power is equal to voltage * current, so 150,000 watts / 250 volts = 600 amps.
Just.. wow. That power level has to be sustainable too, not a peak value, or else we’re gonna blow stuff up from overloading the batteries.
April 18th, 2008 at 12:28 pm
Is there an issue with managing the heat from batteries?
April 18th, 2008 at 1:26 pm
It kinda depends on the battery technology used. Lithium Ion batteries is one of the better commercially available types, but has a tendency to go into thermal run away and explode. Remember all the worry like a year or two ago regarding laptop batteries? That’s why. Just use cooling solutions similar to current cars: lots of surface area and fans for when you’re not moving fast enough. The amount of waste heat created from batteries and electric motors is pretty small compared to what an ICE dumps through the radiator and out the exhaust pipe :P