Acceleration Test: Debunking the Myths
There are two competing ideas about how to use the
least amount of fuel while accelerating a car from standstill to
cruise speed. Conventional wisdom tells us that accelerating slowly
uses less fuel than accelerating quickly. However, others argue
that accelerating a car at wide open throttle to reach cruise speed
uses the least amount of fuel because the time spent accelerating
(using lots of fuel) is much shorter.
So who is right?
The short answer is that conventional
wisdom is mostly right. However, as with most things, the answer
is not so simple. The difference in fuel consumption between a slow
and brisk acceleration is about 0.1% for the same 100 yards (0.06miles).
And because this difference occurs infrequently over a short distance,
there isn't much to gain by accelerating slowly.
For city driving, there might
be benefit to accelerate slowly because of the high frequency of
acceleration from standstill.
The difference in fuel consumption between slow acceleration
and maximum acceleration (wide open throttle) is a little more significant.
Wide open throttle uses 3.7% more fuel than slow acceleration (or
3.6% more than brisk acceleration). This was not entirely due to
the car running rich (using excess fuel). Not on this 0-30mph test
anyway. The car only spent fractions of a second in rich condition.
However, the turbocharger did push the manifold air
pressure up to six pounds per square inch. This large volume of
air is responsible for requiring so much fuel. Of course, this same
volume of air and fuel is also responsible for getting the car to
30mph in such a hurry. And it is also the reason the car was able
to coast for some seconds with the foot off the throttle before
it was necessary to resume part-throttle to maintain 30mph.
Despite the short acceleration and period of coasting,
wide open throttle used 3.7% more fuel than slow acceleration.
Accelerating slowly from a standstill won't result
in a huge or even noticeable gain in gas mileage over accelerating
normally. However, accelerating at wide-open throttle is a sure
way to increase fuel consumption.
Directly measuring the rate of fuel consumption is
not practically feasible as described here.
The most practical way to measure fuel consumption is to calculate
it based on the measured mass of air flowing through the engine.
The air pressure in the manifold, or mass air flow, engine rpm,
is measured by the engine computer. The air to fuel ratio is measured
by a wideband oxygen sensor. The engine data is collected on a data
logger that gathers increments of data many times per second.
The data is organized in a spreadsheet and calculated for mass of
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