Horace Heffner wrote:
At 10:05 AM 3/30/5, Jed Rothwell wrote:
30 hp, by the way, seems a little low even for a lightweight electric car,
based on the performance of my 40 HP Geo Metro. I think you need more like
70 to 100 HP, even with a light, aerodynamic car. The Honda Insight has a
73 hp engine and a 10 kW electric motor. The Toyota Prius has 110 hp,
gasoline-electric combined.
[snip]
Gasoline is roughly 20,000 Btu/lb, and 6 lbs/gal, thus has about 120,000
Btu/gal. A car that gets 30 mi/gallon at 60 mi/hr uses 2 gallons per hour,
or 240,000 Btu/hr = 94 hp. However, if the motor is only 30 percent
efficient, the motor is really only putting out about 21 hp. A car or
motorcycle that cruises at 60 mi/gallon at 30 percent efficiency is using
only about 10 hp to cruise. For some reason I just find this fascinating.
The extra horsepower, which is indeed needed for normal and safe driving,
is really needed primarily for accelerating and hill climbing. This is
obviously one of the reasons hybrids work so well - they can combine energy
sources when a power boost is needed, but cruise on a smaller motor.
I understand that this is how they work but I've never understood why.
Why is it not better to use a gutsy electric motor, a small gasoline or diesel engine, and a battery pack? That always seemed like the "reasonable" way to build a hybrid -- take a tip from diesel electric locomotives, that run the diesel engine at a very efficient constant RPM, and use the electric wheel motors with their very wide dynamic range to do the "impedence match" to the terrain. The battery pack gets you onto the expressway and up the hills, and the petroleum based motor/generator provides all the electricity that you need while cruising.
Obviously my intuition here is wrong. Is there a 20 word explanation of why?
