Improving Your Mileage
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:
- Combustion (the power producing stroke)
Generally, the four cycle engine is the predominant engine used in motor
|A Four Cylinder
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.
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.
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
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
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.