Home Page | About Me | Home Entertainment | Home Entertainment Blog | Politics | Australian Libertarian Society Blog | Disclosures

Know It All: Hybrid Cars -- The Hybrid, or the Half Electric Car

Published in Geare magazine, Issue #54, 2009

It is de rigueur in science fiction novels worthy of the name to have at least one innovative concept or gadget. In one of Robert Heinlein's last novels -- Friday -- his gadget was called the Shipstone, and was simply an advanced storage device for electrical energy. A super-battery, if you like.

Really super. In the novel, a Shiptone installed in a hover car would run it for months. The advantage of a super battery is that it makes shifting energy a lot easier. You can time-shift it (from solar cell capture during the day to night-time use), or physically shift it, as in a battery powered car.

Unfortunately, in the real world we don't have a Shipstone. Nor anything within even a couple of orders of magnitude of its energy-carrying capacity.

If I jumped into a Tesla Roadster -- a state of the art two seat electric sports car costing a mere $US109,000 -- its lithium-ion batteries would be exhausted before 400 km rolled up. And that's using the company's own figures. That's pretty impressive given that the 53kW-h battery carries about the same energy as five litres of petrol.

But when its nearly seven thousand battery cells have run down, it takes over three hours to recharge them again.

By contrast, when I jump into my six year old Falcon after topping it off at the service station, the 'Range' indicator on the trip computer says 560km. When that runs out it takes five minutes to fill up again, ready to go another 560 clicks. And that's taking my whole family, not just the two people permitted by the Tesla.

So it seems a no-brainer: stick with petrol!

But let's run some money-style figures. To go 400km my Falcon consumes about 43 litres of petrol. At a dollar a litre, that's $43. At 13 cents per kilowatt-hour for electricity, the Tesla Roadster would cost less than $7.

One of the ways it does this is by increased efficiency. Electric motors are far more efficient than internal combustion ones, producing far less heat. The Tesla is estimated to be 90% efficient, compared to 15% for a typical petrol powered car. Another is by recovering some of the kinetic energy that regular cars waste. When you step on the brake pedal in your car, its kinetic energy is converted to heat and radiated into the air. In a Tesla, much of it is converted back into charge in the batteries.

The hybrid is a halfway step, intended to gain some of the advantages of a full blown electric car while sidestepping its major disadvantages. In doing so, it introduces some of its own disadvantages. These boil down to the natural results of having two motors to drive one car.

What follows is based on the Toyota Prius, the current leading hybrid in terms of sales.

The Prius has both an electric motor and a petrol one. There will be times when one is dragging around the dead-weight of the other. Despite this, the system offers overall fuel efficiency advantages through a couple of tricks.

First, the petrol engine itself uses a different design, called the 'Atkinson cycle', which connects the cylinders to the crankshaft through levers. These permit the engine to have a longer power stroke than the intake stroke. This reduces power output, but increases efficiency by about 15%. Second, the petrol engine switches off whenever it isn't required. This can be during low speed cruising (if there's enough power in the batteries for the electric motor), or when you're stopped at traffic lights.

Third, kinetic energy is recaptured from the car for reuse through the use of electric generators for braking.

You cannot plug the car into a powerpoint to charge up the batteries that way. Instead they charge from that so-called 'regenerative braking' and a generator driven by the petrol engine. The battery pack holds less than two kilowatt hours of energy, or three per cent of that carried by a Tesla Roadster. It isn't there to run the car for a long time. Instead it provides a power boost when required by the lead-foot in the driver's seat.

The main fuel economy boost compared to a conventional car comes from city driving where the brakes are used a lot. The Prius can harvest much of that braking energy. Highway driving doesn't provide that advantage. The car apparently does provide high fuel economy on the open road, but primarily because of its aerodynamically slippery shape rather than its internal workings.

Toyota is working on a pluggable model, which should appear in a year or two. These are likely to have much bigger battery packs so as to hold a useful amount of energy.

© 2002-2009, Stephen Dawson