Friday, June 17, 2005

On the Fuel Efficiency of Engines

[18th June edit: added index of contents for this long post. They are links that will magically teleport you to the correct paragraph!]

Table of contents:
  • Introduction
  • The 4 strokes of an internal combustion engine
  • Pumping work introduced with example
  • How the throttle affects pumping work
  • Engine revs and power generation potential
  • Engine revs and power absorbtion
  • Normal driving
  • The pros and cons of using a huge engine
  • The fuel economy of a small engine with the power potential of a huge engine
  • Cite this article

  • There are 2 well known ways to save on petrol-
    Use low revs
    Don’t buy cars with big engines.

    Lets justify these pieces of advice. First, we will take a close look at the common internal combustion engine found in motor cars, and then move on from there.

    The internal combustion engine operates by 4 repeating steps (strokes)-
    (a) Induction (sucking in air and fuel)
    (b) Compression
    (c) Combustion
    (d) Exhaust (venting hot air and combustion products)

    Combustion is where the power generation takes place. The air-fuel (A/F) mixture that has been compressed is ignited, which expands and drives the crankshaft to rotate. The other 3 steps absorb power.

    Induction and exhaust use the most power. For an illustration, breathe through your mouth. Close your mouth to leave a tiny gap, as small as possible. Breathe in and out rapidly. The smaller the hole, the more energy needed to pump air in and out, and the faster you tire out. This is sometimes termed the pumping work, which is the work you need to do to pump fluid through a restriction.

    The throttle pedal which your right foot operates is connected to the throttle. This throttle is nothing more than a blockage in the air pipe leading into the engine. It is like a door set in the middle of a corridor. The more you open the door, the more draft (of air and fuel) that comes blowing through.

    In this illustration, the left diagram shows an end view through the throttle body, with the partially open throttle plate visible. The right diagram is a side cut-away view. The partially open throttle plate is visibly inclined. If the plate is vertical, it completely blocks the passage; if horizontal, the throttle is open at maximum, and this full open is also called wide open throttle (WOT).

    Apart from the throttle, other contributors to pumping work are the various flow restrictions such as the air filter, piping lengths, inlet valves, exhaust valves and silencer/ catalytic converter.

    Every portion which the air passes through reduces the air pressure, contributing to pumping work of the engine.

    Engine sizes are usually measured by their displacement. A 2.7 litre engine will suck in 2.7 litres of air every 2 turns of its crank shaft. When it is revving at 4000 revolutions per minute, it will suck in 5400 litres of A/F mixture every minute (4000 x 2.7 / 2 = 5400). After combustion, it will then be vented out through the exhaust pipe.

    Say you are cruising along a stretch of highway when a sudden urge to accelerate overcomes you. What do you do? Drop down one or two gears and floor the pedal, of course! Why drop a gear?

    In general, an engine will produce more power when it is revving faster, up to a limit. When revving at 2500 rpm, a 2.7 litre engine sucks in 3375 litres of A/F mixture every minute; at 6000 rpm, it takes in 8100 litres every minute. Obviously, when it takes in more A/F mixture, it can burn more fuel, hence produce more power. Going one gear down makes the engine rev faster, thus giving the potential to produce more power.

    Descending the steep slopes of Genting or Cameron, one often comes across reminder signs to “guna gear rendah”. Why would we want to use a lower gear? Again, it is to rev the engine faster.

    A fast revving engine sucks in and vents out more air, as already illustrated above. However, if we do not need much power, we only depress the throttle pedal lightly. The throttle is only opened slightly, and the opening for A/F mixture to enter the engine is small. Compare the throttle to your mouth. Open it wide and breather through it quickly. Now, close the throttle (lips) to nothing more than a pin hole, and it is apparent that a great deal of energy needs to be spent just to pump air in through the small opening. It is now obvious (I hope) that an engine can produce more power at high revs, but can also absorb a lot of power at high revs. By using the engine to absorb power via the pumping action at the throttle, one needs not stand on the brakes all the time.

    Most of the time, we do not use cars to race with idiots, nor is it used to go down Genting very frequently. It’s often caught in dense traffic, pottering around at about 60km/h. In these conditions, we are not concerned with power output, nor do we want it to absorb lots of work. What we would like is for the engine to produce the required amount of power using the least amount of petrol. The throttle pedal would probably be depressed less than 1/6 the way down, and the throttle itself would be only open partially to allow a little A/F mixture into the engine to produce a little (just enough) power.

    If we use a low gear, the engine will rev quickly, and thus suck in more air. Remember that the throttle is only open partially; hence sucking a lot of air through it will need lots of energy. This pumping work is absorbed from the engine, which converts energy from fuel into work output at the driveshaft. The conclusion: pumping work is a waste of petrol (good money). Use a higher gear, and the engine revs slower, sucking in less A/F mixture, resulting in less pumping work.

    Return again to the mouth-throttle comparison. Close your mouth to a small gap, and suck air in s.l.o.w.l.y. Now, maintain the same throttle opening, but suck fast. You will notice that the force required to suck air in slowly is considerable less than a high flow. If you do not faint from a lack of oxygen, you can do it slowly all day without getting tired.

    If we replace the 2.7 litre engine with a monster 5.5 litre thing, it is obvious that it can produce more power than the 2.7 unit. But it can also absorb more power than the 2.7 unit. At 1500 rpm, they each take in 2025 and 4125 litres of A/F mixture every minute. At 6000 rpm, the figure rises to 8100 and 16500 litres per minute, respectively. If we were cruising at 60 km/h in dense traffic, both only need to produce enough power to over come friction to move our car at 60km/h. However, the 5.5 litre engine needs to suck in much more air through the throttle compared to the 2.7 litre engine. Hence, the 5.5 litre car needs more petrol to sort out the pumping work.

    Thus, we see that the 2 bits of advice mentioned earlier are indeed justified.

    And for the best of both worlds...

    What if we need the power potential of a large engine and the cruising economy of a small unit? General Motors have made available a technology called Displacement on Demand (DoD), while Honda has developed Variable Cylinder Management. Both these technologies shut off half of the engine’s cylinders during low load operation by not opening the intake and exhaust valves of these cylinders. With air trapped in these cylinders, they behave like springs, absorbing work, and then returning most of it with every rotation of the crank shaft. This means that only half the air is being sucked into the engine through a constricted throttle, and the pumping work is reduced significantly.

    Cite this article as:

    Tan Yee Wei (2005), "On the Fuel Efficiency of Engines", from "Snippets of This and That".

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