Saturday, August 08, 2009

Almost time for a new car

Our minivan has hauled our dogs, kids, and gear for almost 9 years, and it's starting to show. In another couple of years, we'll need a new car. So, if you're building cars and wondering what to build next, let me tell you what we want.

The last time I knew I was going to buy a new car, I wrote a letter to Chrysler two years ahead of time. Fat lot of good that did. This time, I'm asking for essentially the same thing. I'll post it on my blog instead.

We want a plug-in hybrid minivan. Plug-in hybrids face a couple of big problems: the batteries are too heavy and the engine runs intermittently, which prevents the catalyst from firing up and leads to nasty emissions. I think both these problems are completely solvable for a practical vehicle that we would buy in a heartbeat.

First, I'll point out that 1100 pounds of lead-acid batteries can store 16 kilowatt-hours, which is the government's definition of an electric vehicle. Those batteries can survive five years of cycles through 30% of their capacity. 4.8 kilowatt-hours is enough to push a minivan 13 miles. That's less than the average daily drive of 33 miles, but for a minivan used for multiple short trips a day, it's easily good enough.

Next, I'll point out that the emissions problems can be solved by delaying the first ignition of the engine. If the minivan can get to 50 MPH on batteries alone, then it can avoid the engine everywhere but on the freeway. For most trips our miniman makes, that means no engine at all for most trips, and that basically eliminates the emissions problem.

Finally, I'll point out that regenerative braking extends the EV range just a bit, and comes with a lot of complexity (control interaction with the friction brakes) and cost (fancy controllers). I'd certainly be willing to live without it if it cost $1000 and only got me an extra mile of range.

Here's what the minivan would look like:
  • Packaging
    • It should have seating for 7: 2+2+3.
    • It should carry many 4' x 8' sheets of plywood in the back.
    • It should have two sliding side doors, etc, just like real minivans.
    • Seats do not have to stow. They can come out like my current minivan's seats do.
    • Including battery pack, it should weigh 5200 pounds. That sounds like a main battle tank, but it's pretty reasonable once you think about the 1100 pound battery pack.
    • Weight distribution should be close to 50:50 front:rear, and the center of mass should be very low, so the thing should handle reasonably well.
    • The thing should be quiet when driving in EV mode.
    • It should plug into a normal 3 prong 120V AC outlet.
  • Performance
    • 0 to 60 in 10 seconds. (Requires an average of 115 wheel HP.)
    • 0 to 60 in 20 seconds on batteries. (Requires an average of 57 wheel HP.)
    • 75 MPH up a 6% grade with 1000 pound load. (Requires 77 wheel HP, plus whatever is needed to go in a straight line at 75 MPH. 115 HP ought to do.)
    • Maximum cargo load of 1400 pounds.
    • It should go 13 miles on a 30% cycle of the batteries.
    • It should go 350 miles on a full tank.
    • EV mode should work: the car should be able to cool a hot interior and get to 50 MPH without starting the gasoline motor.
    • 20 MPG from the gas engine alone. That's about 4000 joules/meter of gasoline energy, or 15 cents/mile for gasoline at $3.00/gallon.
    • It should use about 800 joules/meter of battery energy. That's about 4 cents per mile for the electricity, at the average US residential rate (10.5 cents/kWh).
    • The batteries should charge from 70% to 90% in 90 minutes from a standard plug.
    • The batteries should charge through a 30% cycle in 5 hours.
  • Drivetrain
    • It should have front-wheel drive from the gas engine.
    • The gas engine should be a 2 liter 4 cylinder engine with around 130 horsepower. That sounds anemic, but add 60 electric horsepower and it's a whomping 180 HP.
    • It should have rear-wheel drive from the electric motors.
    • The motors should deliver 60 horsepower at 30 MPH (torque limited below). This will give excellent performance in deep snow over pavement.
    • The electric motors can have their torque die to nothing at 70 MPH. Any faster and the gas engine is required anyway.
    • It should have about 1100 pounds of lead-acid batteries, which deliver 17.5 megajoules with a 30% cycle. This just hits the 58 megajoule full-cycle battery that the US government is willing to subsidize as an electric vehicle -- $7500!
    • It should have a 330 watt solar panel covering most of the roof. This sounds silly but it's actually a good idea. The panel adds about 4 miles of electric range on an average day in California, at almost the same cost per mile of range as the battery, with very little weight.
  • Cost
    • The thing will go 17.5 miles a day in EV mode if charged only at night and parked in the sun, and 27 miles a day if charged at work as well. If used as a daily driver, it'll cover 6,000 to 9,000 miles a year in EV mode.
    • It will save around 5 or 6 cents per mile. Obviously, that's not why people would buy it, but it does make for $300 to $500 saved each year.
    • The roof panel will cost about $1200.
    • Battery swap costs $1800 (half of the new cost). Batteries should last 5 years, or 1800 30% cycles, so that they cost 6 cents/mile. Existing lead-acid batteries already achieve this cost.
    • The added cost will take 10 to 15 years to pay back (if you ignore the subsidy).
I think the drivetrain can be a lot simpler than a Prius drivetrain. In particular:
  • The electric motor/generator on the gas engine doesn't need to be big. It needs to be big enough to start the engine quickly (maybe 10 horsepower), and that's about it. I don't want to recharge the batteries from the engine any faster than 10 HP anyway. Gasoline costs 3 times as much as electricity from the plug, so the only reason to charge the batteries with the engine is if I can avoid starting the engine later in the same trip.
  • Make sure the heater and air conditioner can run off the batteries. It's important that these be able to run right at the beginning of a trip without having to turn on the gas motor.
  • Lead-acid batteries. Forget the fancy batteries. Even lead-acid batteries cost more than the electricity from the plug costs, other batteries are worse. Lead-acid can deliver the necessary range and power without the availability headaches of NiMH or Li-ions.
I think that roof-mounted solar panel deserves some explanation. It has a lot of interesting benefits:
  • On a sedan, there wouldn't be enough roof area to make a significant solar panel. A minivan, on the other hand, has a pretty big roof, so the idea works better.
  • The car can run its fans continuously when unattended. When you get to your car sitting in the parking lot in Phoenix, it doesn't hurt to sit down or touch the steering wheel. The interior won't disintegrate in the extreme heat either.
  • When the ignition is turned off, it should be possible to turn on the A/C and get 70 cfm of air cooled by 40 F. That's enough to turn over the car's air every 2.5 minutes. It won't cool a car that's already gotten to baking temperature in the sun, but it will keep a car cool after you turn it off.
  • Batteries don't like to be run down for long periods. With a solar charger, the car will get some juice every day, which can keep the battery topped up if you leave the car unattended for a while. This will improve the battery life, and it's just nice to come back to a car and have it fully juiced.
In our family, Martha would drive this thing, using it primarily to move the kids around. She makes multiple short trips each day, usually not on the freeway, so it would get plugged in regularly and probably only use gas for the trips to my parent's house, which is 55 miles each way. Since we'd get plugged in while at their house, the minivan would end up driving 80 miles on gas, on the freeway, where it gets 26 MPG. If we do twenty trips like that a year, we'd burn just four tanks of gas and our overall gas mileage would be 195 miles per gallon of gasoline.

"When did you last fill up?"


Seems like a winner to me.


  1. Yup, this is what I was looking for when I bought my last car 4 years ago. Closest thing I could get to it then was a Prius (which I bought). And in the next couple of years we'll be looking for a family car (minivan/suv), so these specs would be ideal. We'll take one!

  2. Hi, nice post. I have been pondering this topic,so thanks for writing. I will probably be coming back to your site. Keep up great writing.