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Why you can’t have a super economy car   Leave a comment

From time to time, I hear about this car: http://www.seattlepi.com/news/article/Hybrids-meet-your-rival-it-gets-376-59-mpg-1264903.php

It’s an old Opel that got 376.59 miles per gallon.  And people will say “OMG, why didn’t they make more them?  Why didn’t they turn those out at the factory?  It’s a conspiracy, man!

I’m going to break this down in the simplest terms possible:

The physics of better economy.

(1.) The harder you push air, the harder it pushes back.  If you really, really want to improve your economy, slow down. Half the speed, and you square root the power required.  If it takes 100 hp to go 100 mph, then it takes 10 hp to 50, and 3.16 hp to go 25.   Below about 30, aerodynamics effects are pretty minimal.

(2.) Cars get better economy in higher gears, so you have to set up the gearing so that the car is at it’s top speed when it is still going slow.

(3.) All otto cycle (the normal, nondiesel kind of car engine) produce the most power per lb of gas consumed at around 900 – 1100 piston feet per minute.  The smaller the engine, the higher RPM that will occur at, but it will be  almost always be lower than your normal cruising speed.

(4.) Energy that isn’t moving the car forward is eating fuel economy.

(5.) Weight that isn’t moving the car is eating fuel economy.

Knowing that, how do make a super mileage car?

We make it as narrow as possible so it is pushing on as little air as possible.  Then make it low and torpedo-like so it wastes as little energy as possible moving the air it must.  Gear it so it is in 5th gear when it is doing 30 mph or less.  Maximize the engine design to produce power only in the narrowest efficiency band.  Eliminate any extra parts that move: replace the transmission with a single straight-through gear. Take out as much weight as possible.

Makes sense right?  And what did the designers of the Opel do?

They attached Opel body panels to a big go kart frame.  It sat low, had no suspension, no transmission, no seats, heater, no air conditioning, no anything.  It was chair in a go kart frame.  The engine was connected to solid rubber tires (no flexing rubber to eat energy) which turned locked together, without a differential (no 5% loss in the diffy).  The engine was insulated so that as much of the heat that was lost into the coolant could be recycled to heat the intake instead (the gas was heated for the same reason: to take energy out of the exhaust and put it back in to be recycled.   Above all, it had a top speed of 30 mph.

Why don’t we do those things now?

(1.) We do.

http://technorati.com/lifestyle/autos/article/eco-marathon-of-future-cars-3000/

Off-the-shelf technology exists to get not 50, not 100, not 377 mpg, but a full on 3000 mpg.  You have to wear the car like body sock, go 18 mph, and sit inches of the ground.

Why don’t we do this more often?

(1.) Above all, lack of interest.

People don’t really want better economy.  They really don’t.  When people say “I wish I got better gas mileage” an silent “without changing my lifestyle, including using the level of consumption I can afford as a status symbol, at all” follows it.  In the Ford Falcon of the 60’s, the bigger six outsold the smaller one by a wide margin.  The Chevy II’s little 4 cylinder was rarely purchased, and dropped quickly.  V8 mustangs outsold I6 mustangs by a 14:1 ratio.  You can’t say that those numbers had anything to do with the perceived lack of safety in small cars, because those were simply different engines in the same cars.  That wasn’t simply pre OPEC hubris either.  Chevrolet introduced the Chevette in 1979, with 2 engine choices: a 85 and 98 ci.  The 85 ci engine was dropped within a few years because so few people bought it.  In 2011, Ford Focuses outsold Ford Fiestas by a vast margin, despite the fact the Fiesta has better handling, gets better gas mileage, and is one of the safest cars on the road.

(2.) Mother Nature and her red headed step child, economics, will not be cheated.

In each of the following lists you can have only 2 of 3.

Strong, light, cheap

Aerodynamic, durable, cheap

Fuel efficient, broad power band, cheap

Functional, stylish, cheap

Safe,  small, cheap

Safe, light, cheap

Economy, convient, cheap

Economy, comfortable, cheap

(3.) In the end, pollution and the cost of commodities (like cars and gasoline) are not technological problems.  They are social ones.

 

 

 

 

 

 

Posted April 26, 2012 by israelkwalker in Uncategorized

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