This Idea Is Now OLD NEWS

On 24 May 2002, Volkswagen announced that it had built a two-seater that gets 235 MPG See Popular Science. VW's Lupo has been getting about 80 MPG for some time now. So this project turned out to be much easier and quicker than I had anticipated. Problem solved, more or less. Now all we have to do is get the SUVs off the road....  

This page was in draft form on 22 August 2001. It will not be updated.  

Introduction

Objective

The objective of this proposal is to reduce the per-mile energy consumption of cars by a substantial amount. A goal of 100 MPG within 10 years is proposed. We'll call it the "C-car" because of its 100 MPG fuel economy target. The C-car is a four-seater with moderate baggage storage and is probably arranged as a hatchback. It's worth mentioning here that Amory Lovins of the Rocky Mountain Institute has proposed a "Hypercar" that gets 200 or more MPG. The Hypercar concept is prsented on the Rocky Mountain Institute's web site

State of the Art

We can compare the C-car to the late 1970s Volkswagen Rabbit Diesel. This car had 49 HP, was capable of about 80 MPH, and got 45-50 MPG.

The Lovins Hypercar is a far more sophisticated design that what I propose. While some elements of his Hypercar are speculative, the C-car can be built with existing technology. The other really important difference is that Lovins begins with the assumption that the Hypercar must perform substantially as well as modern automobiles. The C-car is based on a top speed of 45 (maybe 50) MPH and modest acceleration. This can only be achieved, of course, if national speed limits are set at 45 MPH and really enforced. If energy prices rise sufficiently, this will not be a difficult goal to attain.

A General Strategy for Reducing Energy Consumed by Cars

Much lower speed limits

• Square-law relationship between speed and aerodynamic resistance
• Super-proportional increases in rolling resistance with speed (about 1.5 x resistance as speed doubles, for a 3-fold increase in power required)

Reduce annual vehicle miles traveled

The use of cars will be discouraged in various ways, including much higher fuel prices, considerably lower speed limits, and an end to all motoring subsidies, which will approximately double the cost of driving one mile, to around US$1.00. (The actual increase will be more than 100% as the cost of petroleum rises.)

Improving vehicle-mile fuel consumption

The C-car would get nearly four times as many miles per gallon as modern automobiles, which, in the USA, have a fleet-weighted fuel economy of about 27.5 MPG (excluding passenger trucks such as the Ford Explorer, which are supposed to get 20.7 MPG as a group).

The Energy-Efficient C-Car

In this section, we consider ways that the C-car can attain the 100 MPG design standard. This is not as difficult as it may sound, as the energy consumption of cars on a seat-mile basis is now extravagant. There are several reasons for this:

• Highly inefficient propulsion systems
• Exceptionally high rolling resistance
• Large frontal area in comparison to the size of the vehicle
• Poor aerodynamic shape
• Excessive weight

We will consider each of these issues in turn

Efficient propulsion system

Many modern cars have 200 HP engines that are capable of rocketing the car to 60 MPH in well under 10 seconds and can give a top speed of 130 MPH or more. The result of this, however, is that at normal highway speeds, the engine is producing only around 35 HP, and engine efficiency is poor at this power loading. The weight of the engine and drive train is far higher than it would otherwise have to be. The energy cost of driving a car capable of tire-screeching acceleration and race-track top speeds is very high, even if the driver never floors the accelerator and always abides by speed limits. In addition, these huge engines require large radiators to dissipate waste heat, and the radiators themselves are major causes of aerodynamic drag.

The C-car won't go faster than 45 (at most, 50) MPH. Acceleration can be sedate, since the highway speeds will be reduced from a de facto 75 MPH today to 45 MPH; tremendous acceleration is no longer required to safely merge into Interstate traffic. Until the 1954 model year, the Volkswage Beetle had only a 25 HP engine. I don't know the top speed of the 25 HP Beetles, but it must have been about 60 MPH.

I estimate that 20 HP is ample for the C-car, which compares to the 25 HP Beetle as follows:

• Far more efficient power plant
• Lower-resistance tires
• Somewhat less frontal area
• Much better streamlined
• Several hundred pounds lighter

New techniques are becoming available to clean up the filthy exhaust from diesel engines, probably making them the engine of choice for fuel-efficient cars because of their inherently better fuel efficiency.

The engine for the C-car is probably a two-cylinder, four stroke, horizontally opposed, water-cooled 20 HP diesel. At full power, such an engine should consume about 8 pounds of fuel per hour, or just over one gallon. The intention is that the engine would not run over 45% power on straight-and-level roads at 45 MPH, giving about 100 MPG.

The Model T had a 20-22.5 HP engine (of nearly 3 litre displacement!), two gears, and a top speed of 55 MPH. It weighed 1200 pounds and rode on 30-inch tires. It's reputed to have been quite quiet in operation and to provide a fairly good ride despite the simple suspension.

Reduced rolling resistance

According to Marks Handbook, a rubber tire has nine times the rolling resistance of a steel wheel on a steel rail; when the tire is cold, the figure is considerably worse. Nothing can ever eliminate this discrepancy, but several strategies can reduce it:

• Large diameter tires
• High tire pressure
• Tire designs with reduced hysteresis
• Tires compounded with low-rolling-resistance rubbers
• Low-profile tires (which have reduced losses to sidewall deflection)

Notice the tires on racing bicycles. They are as large as the rider and frame permit, they have a very small cross-sectional area, and they are inflated to very high pressures. This is all done in the interest of reduced rolling resistance. Any bicycle pedals more easily when the tires are well inflated.

The newest radial tires have only half the rolling resistance of old bias-ply designs. Radials are now standard equipment on most cars. While there are many trade-offs in tire and vehicle design, I would guess that tire diameter will remain roughly the same as it now is on regular passenger cars. The size of the wheel will be increased, yielding a low-profile tire with a tread width about the same as today. Tire pressures might be double those of today, resulting in a harder ride but considerably lower rolling resistance. The lower speeds will compensate for the harsher ride.

Somewhat reduced frontal area

In comparison with a train, the relationship between the frontal area of the vehicle and its size is highly unfavorable, giving rise to extremely high aerodynamic resistance. (The "Reynolds Number" is very unfavorable for short vehicles with large frontal area.)

The "econo-boxes" of the 1980s had comparatively small frontal area. These cars were no wider than required to seat two people side by side, and no higher than necessary. (The SUVs now so popular are about as wide as cars have ever been made, and much taller, so their frontal area is about as great as can be imagined.) By lowering the seating position, the height of the car can be reduced somewhat. This is about the only gain I foresee.

Reduced aerodynamic resistance

Cars are considerably cleaner in aerodynamic terms than 25 years ago, but further improvements can be made, especially as the car will only operate at modest speeds; no thought need be given to road-handling aerodynamics at high speeds, eliminating the need for devices to increase down force.

The bottoms of most cars are very rough, and the addition of a simple bottom pan can considerably clean up the air flow.

Greatly reduced weight

As the car becomes lighter, all of the components can become lighter, which is a benign circle. For example, in the C-car, a 3-gallon fuel tank gives 300 miles or 6 hours range. Not only is the tank itself considerably lighter, but the average weight of the fuel is very considerably reduced. An efficient 20 HP diesel engine might weigh just 20% as much as a conventional 200 HP gasoline engine (diesels are heavier per HP than gasoline engines). As vehicle weight declines, tires and suspension can become lighter, so the loads imposed on the (presumably monococque) chassis are reduced, allowing a reduction in chassis weight. It should be possible to build a C-car with a curb weight that does not exceed 1000 pounds, perhaps considerably less.

Reduced weight translates directly to fuel economy. The power required to accelerate the car is reduced in proportion to the weight, as are the rolling resistance and the power required to climb hills.

Copyright © 2001 J. Crawford. This page may be freely reproduced provided that J.H. Crawford is acknowledged as the author, his copyright is maintained, and the full text is reproduced.

By publication of this work, J. Crawford places in the public domain any original ideas contained in the work. 2001-06-29