INTRODUCTION
Starting torque sells cars. Almost every driver is favorably impressed during a test drive of a new car he is considering purchasing when the gas pedal is floored and he is pressed rearward in the seat.  A wimpy acceleration will usually kill a deal.

Acceleration must be able to match that of of conventional cars if the driver of an electric car is going to keep up with traffic.  Early hobbyists who converted cars used available, economic, hunt-wound aircraft-surplus type starter-generators.  Their cars were plagued with slow acceleration and horn-honking by drivers following them.

Electric car range and acceleration will be covered in this Series of Essays.  Part I will start off with an overview of range considerations where fundamentals for powering a passenger vehicle will be presented.  We will deal with drag forces that reduces range.  next will come the fundamentals affecting acceleration.  In Part II sample calculations will be applied to various components.  this will be continued in Part IV. Part V will consider battery characteristics. Part VI will examine battery chargers. Part VII will take a look at currently available components for conversion work.

RANGE
Over twenty years of experience has been accumulated with lead-acid batteries and substitutes that included nickel-iron, nickel metal hydride, sodium-sulfur, nickle-cadmium, and a host of other elctrochemical combinations.  These substitutes can store 2-3 times the energy of a lead-acid battery, but are many times more expensive.

No battery can approach the energy-storage capability of gasoline.  A pound of gasoline stores 115,000BTU's of energy that is used by an engine with an efficiency of about 10%. That leaves 11,500 BTU's to move the car.  The usual range for a converted car using a lead-acid battery pack and driving in the usual urban traffic is less than 50 miles.  There have been "tests" that report longer distances, but they are driven at a constant speed, usually 25-40 mph, on a test track.  Be skeptical of range claims and find out what were the test conditions.

The Federal government and private industry has spent millions of dollars trying to produce an electric car with a range equivalent to a gasoline car.  Forget it; an examination of the electrochemical series of candidate battery materials will show this is a quixotic quest.

Persons who have converted cars to electric drive have learned to use them within their range limitation.  Car manufacturers don't believe this approach will be acceptable to the public. What is needed is a good public relations campaign, ala the original VW Beetle ads, informing the public that an electric car makes environmental and economic sense.

PERFORMANCE FACTORS
Six factors are involved in fixing the performance of a passenger car.  They are; weight, rolling resistance, air resistance, road gradient from horizontal, and propulsion effort from the power source and transmission.

Weight
The most important factor defining vehicle performance is vehicle weight.  Most conversions use 12-15 batteries.  These can be six, eight, or twelve volt modules, each weighing about 70 pounds.  Twelve batteries weigh 840 pounds, fifteen will weigh a half-ton.  Battery weight makes up a maximum of 30% of total converted vehicle weight. The curb weight of a typical conversion vehicle will be 2,800 to 2,500 pounds.  For convenience, 3,200 pounds will be used for illustration purposes.  The ideal weight distribution is a 50-50 split between front and rear wheels.

The following Table gives the weights for vehicles owned by FVEAA members:

YEAR	Mfg - MODEL	Allowed	Converted	L. Front	R. Front	L. Rear	R. Rear
1998	Ford Ranger*	5467	4816	1239	1178	1189	1205
1986	Dodge Omni	3284	2212	752	655	383	422
1990	Nissan Sentra	3117	2971	639	613	877	842
1980	Mazda RX-7	3000	2859	715	714	716	714
	Unique Electrek		2650
	MI Towncar		2700
	Citicar*		1580

* As-manufactured. Others are conversions or modified

Road Load
This is the name given to the rolling resistance caused by friction in bearings and gears plus tires. The flexing of tires under load is the principal element of road load.  Road Load is independent of velocity and directly proportional to weight.  It typically ranges from 0.5-2% of weight.  One way to measure road load is by coast down.  In this, the car is allowed to coast in neutral at low speeds on a smooth, level surface.  Observe the initial speed, the final speed, and the elapsed coast-down time. The Road Load coefficient can then be calculated:

Kr = Initial Speed - Final Speed                             (Feet per second)
        (Gravity Acceleration)(Elapsed time in seconds)

For illustration a 3200 pound will be assumed.  It coasts from 20 to 10 mph (29.3 to 14.7 feet per second) in 30 seconds

Kr = 29.3 - 14.7             =        14.6          =   0.015
        (32)(30)                            (96)

The Road Load Force (Fr) for the car is (Kr)(Car Weight) = (0.015)(3200) = 48 pounds

Aerodynamic Drag
The drag caused when a car is pushed thru the air. It increases as frontal area becomes larger and as the square of velocity.  The cars shape is an important factor.  The easiest way to determine this is to get it from the manufacturer's literature.  You can also attach a spring scale to a tow line and tow it at various speeds on a smooth, level surface.  The force is read from the spring scale at various speeds. Subtracting Road Load will yield drag force.

Drag force isn't much of a factor at usual urban speeds up to about 30 mph as the following table for a 3200 pound car having a frontal area of 10 square feet shows:

Speed in Miles per Hour
  20          30          40          50           60
Fd in Pounds          12          27          48          75         180                            

Hill Climbing
Around Chicago this is not an important factor.  There are occasions, however, when it must be considered.  Assume our car is on a parking garage ramp with a 30 degree slope.  The force required to move the car up the slope can be calculated:

Fh in Pounds = (Car weight) (Sine of inclination Angle)
3200 sine 30 = (3200)(.05) = 1600

For a 2% maximum grade on the Interstate System, the hill climbing force would be 112 pounds.

The sum of all the forces at the wheel surface for our typical car travelling at a constant 30 mph would be:

Fr + Fd = Fh = 48 + 48 +112 = 208 pounds

Acceleration
Acceleration is defined by Newton's Second Law of Motion:

ACCELERATION = FORCE
                                   MASS

Before we can plug the car weight into Newton's Equation the proper units must be used. Dividing the weight by the acceleration of gravity, G, (32 feet per sec-sec) gives the car's mass.

MASS = 3200 Pounds                 =          100 Poundalls
               32 Feet per sec-sec

For an electric car to keep up with urban traffic, it should be able to equal their acceleration capability.  Assume you wish your converted car to go 0-30 in three seconds.  This, however gives only one part of the story.  We need to calculate the distance travelled in this time.  Assuming constant torque during acceleration, a close approximate calculation for distance is:

DISTANCE = (0.5)(TIME)(FINAL VELOCITY)            
                    = (0.5)(3)(30MPH)                                        
                    = 45 feet, About the width of a typical city lot

The average acceleration is ten feet per sec-sec.

Volumetric considerations are also important.  Where will you place the batteries?  The ideal weight distribution is one equally divided between the front and rear wheels.  There is limited space in which to install batteries and make connections.  There are two usual locations for batteries; in front after the engine is removed and an an electric motor is in-place, connected to the transmission.

If you put five batteries in the front under the hood and another seven in the rear trunk space over the rear axle this will give the desired 50-50 front-rear weight distribution.  Car handling is affected by a polar moment of inertia where the car tends to rotate about its vertical center point when you abruptly turn.

Another safety factor is the height at which the batteries are installed.  They should be kept low to minimize vehicle roll-over if a sharp turn to avoid a collision is made.  You can put a battery box in the rear space formerly occupied by the spare tire and gas tank, maintaining sufficient road clearance height so the box will not drag when crossing a driveway gutter.  Then you have to decide where the spare tire will be located.

A final note; batteries require maintenance.  The standard lead-acid battery requires periodic watering so the cell caps should be easy to reach.

Bill Shafer
Newsletter Editor
August 20, 1999