Improving Your Mileage

Understand your Engine Understand your Vehicle Understand your Mileage Measuring your Efforts Cost Effectiveness
The Four Cycle Engine Gears & Wheels Mileage vs Gear Vavg vs mpg miles / $  (mp$)
A Four Cylinder Engine Force at the Wheel per gear Acceleration vs Mileage Upper & Lower Bounds THEN
(20 years ago)
The Engine Power Curve   Hyper-Mileage   NOW
        Then vs Now

Understand your Engine

The Four Cycle Engine
In the animated GIF to the right is depicted the four-cycle combustion process (also often referred to as a 4-stroke process. 

One "cycle" (or stroke) is a half-turn (180 degrees) of the crankshaft.  All four cycles take two revolutions of the crankshaft.
The four cycles are:
 - Intake
 - Compression
 - Combustion (the power producing stroke)
 - Exhaust,

Generally, the four cycle engine is the predominant engine used in motor vehicles.


A Four Cylinder Engine
In the animated GIF to the right is depicted a four-cylinder engine. 
Each cylinder also uses four-cycles to produce power.

A four cylinder engine produces a power (combustion) stroke from one of its cylinders every "cycle" (180 degree turn of the crankshaft.)

Motor vehicle engines in cars and light trucks tend to be either 4 cylinders, 6 cylinders, or 8 cylinders. 

The Power Curve
To the right is a graph of the important performance parameters of a typical vehicle engine.  It plots engine torque, horsepower, and fuel consumption against engine speed in revolutions per minute (rpm). 

While your vehicle's owner's manual probably does not have such a graph in it, you can be assured that the engineers who designed and manufactured your vehicle's engine produced one, and studied it very carefully.

Torque (the blue line) is the moment (force), in foot-pounds, that the engine produces at each rpm level.  This is the curve that measures how much force the engine can produce, and is directly responsible for the vehicle acceleration.

Horsepower (the purple line) is the power that the engine makes at each rpm level.  Power is the amount of energy available per unit of time.  One horsepower is exactly 550 pounds moved 1 linear foot in 1 second. 

We can convert foot-pounds per second to foot-pounds per minute by multiplying by 60 seconds per minute.  And we throw 2 x pi in when we deal with rotating engines.  So in terms of Torque in foot-pounds (ft-lbs) and revolutions per minute (rpm)

1 Hp = 5252 ft-lbs/rpm

So if our engine is making 150 ft-lbs of torque at 1,000 rpm, then the power it is making is

Power = 150 ft-lbs x 1,000 rpm / 5252 = 28+ Hp

At 4,000 rpm, and 240 ft-lbs, the power is

Power = 240 ft-lbs x 4,000 rpm / 5252 = 182+ Hp

Fuel Consumption (the red line) is measured in pounds per hour (pph).   A gallon of gasoline weighs 6 lbs, and a gallon of diesel weighs about 7 lbs.

The engine operating limits are the idle speed on the low end, and the red-line at the top end.  The idle is set so the engine won't stall at no load.  The red line is a safe limit under which the engine won't self-destruct.

Note that the rate of fuel consumption increases with increasing rpm. 

Maintain Your Engine
Whether you want to maximize the power available, or maximize its fuel efficiency, regular engine maintenance is required.  Your vehicle has a Owner's Manual with recommended intervals for replacing the air cleaner, oil filter, oil,  transmission fluid, spark plugs, radiator fluid, greasing joints, etc.   Follow that technical advice religiously. 

Keep a special  eye on the coolant temperature gauge and the oil pressure gauge.

A lack of oil pressure is a very serious issue -- shut your engine down immediately to avoid catastrophic failure -- and have your vehicle towed to a repair shop. 

A rising coolant temperature gauge indicates a loss of coolant.  Replenish the coolant before continuing, and take the vehicle into a repair shop if the temperature continues to rise.  If the temperature reaches the red arc or line, shut the engine and don't drive it until the cause of fluid loss is discovered and repaired. 

While you can perform much of the periodic maintenance yourself, there are repairs and maintenance requirements that are beyond the skills and capabilities of the ordinary owner, and / or require specialized equipment or tools.  Examples of this are engine problem diagnosis (the engine light is on) and manufacturer's recalls. 


Typical Engine Maintenance Schedule
(Click on thumbnail, above, to see expanded view)

2014, Simon R. Mouer III, PhD, PE 
All rights reserved.