It’s well known that one of the greatest limiting factors in achieving good fuel economy is the driver. Yes, in real-world use, the person who is steering and operating the brakes and throttle has a dramatic influence on fuel economy.
And we’re not just talking about those people who cane the car everywhere, foot mashed to the floor. Even gentle driving can give poorer fuel economy than is possible. Huh – how can that be? In short, because an engine is much less efficient when it is operating with the throttle only just cracked open.
Let's take a look at this idea.
Engine Fuel Economy
Examine this graph. What it shows on the vertical axis is the amount of fuel that is used to produce each horsepower for an hour. Obviously, the lower the fuel used per horsepower/hour, the better the fuel economy of the engine. Across the bottom axis is engine speed in rpm.
Look at the line highlighted with the green arrow. This shows the amount of fuel used per horsepower/hour at different rpm – measured at full throttle.
Now look at the line indicated by the red arrow. The fuel consumption per horsepower/hour is way higher – typically over double. Why? Because the engine is now working at only 25 per cent throttle.
There’s another way of showing this characteristic of throttled engines. This diagram shows engine load up the vertical axis, and engine rpm on the horizontal. The red area on the diagram is the ‘island’ of best fuel economy. As can be seen, the best fuel economy (measured in terms of fuel used per horsepower/hour) occurs at relatively high loads and low rpm.
The key point to remember is: lowest specific fuel consumption occurs at high throttle angles and low rpm.
To put this another way, driving around with the throttle only just a little open at high engine revs gives much worse fuel consumption than using lots of throttle and low revs.
More to it...
But there’s more to it than that.
When the throttle is floored, nearly all engines add more fuel than usual. That is, instead of adding a ‘normal’ amount of fuel to the air, at full throttle they squirt in some extra. This allows the engine to develop greater power. Therefore, despite what was said above, running at full throttle (eg when going up through the gears) will usually give worse fuel consumption. So a lot of throttle is good – but not full throttle.
Next, nearly all engines have another important characteristic – they turn off the fuel supply completely (ie switch off the injectors) when the throttle is lifted on the over-run. And of course, if the car is moving and no fuel at all is being injected, then the fuel economy is just great! So wherever possible, the accelerator pedal should be fully lifted.
Finally, it’s not possible to drive everywhere with the engine operating in its most fuel-efficient area of operation – the trick is to keep the engine operating fuel-efficiently as much as possible. In other words, real-world driving is about “swings and roundabouts” – eg about using more power, produced more fuel-efficiently, to climb a hill so that the car can then roll down the other side, throttle closed and using no fuel.
OK, so let’s take a look at what we’ve covered.
1. Most engines develop their best fuel economy when given lots of throttle (but not full throttle) and engine revs are low.
2. When decelerating, the throttle should be fully lifted.
3. The worse driving style for fuel economy is a slightly open throttle and high revs.
Now hypermilers (those who chase absolute best fuel economy) have known about these ideas for ages. They train themselves to drive economically by especially avoiding condition #3 above – driving with a slightly open throttle and high revs.
But what happens when you’re doing say 60 km/h in free-flowing traffic? How can you then be in that red island of best fuel efficiency shown in the diagram repeated at right? The way hypermilers do it is to ‘pulse and glide’ – accelerate in the island of best fuel efficiency by using lots of throttle in as tall a gear as possible (#1 above), and then back right off (#2 above). They then repeat the process. From a fuel efficiency perspective, you can see exactly how it works.
So, all well and good. But how does this tech info translate into driving? The trouble is – very often it doesn’t. By the time you’re observing speed limit signs, being distracted by the screaming kids and watching out for kamikaze drivers, the last thing you can concentrate on is changing gears at the right moment - or using the right throttle settings - for best fuel economy.
And that’s where FuelSmart comes in. FuelSmart is an adjustable electronic module that constantly watches two engine management signals.
The first signal it monitors is throttle position – as detected by the standard throttle position sensor (TPS). The module is calibrated so that it can detect when the throttle is open, even if it is open only a small amount.
The second signal it watches is manifold pressure, as measured by the MAP sensor – and this one needs a little more explanation.
The engine is a pump, drawing air in, mixing it with fuel, combusting the mix and then pushing the exhaust gases out. The throttle blade limits how much air can get into the ‘pump’ – the engine. If the engine is running with the throttle nearly closed, a low pressure (a ‘vacuum’) will be developed in the intake system. If the throttle is fully open, the pressure in the intake system will be much the same as the pressure of the atmosphere – there won’t be any vacuum.
The degree of vacuum in the intake depends on two things – how far the throttle is open, and how fast the engine is spinning. (The faster the engine is turning, the more air it is trying to draw in.)
Maximum vacuum occurs on high rpm over-run, when the throttle is shut and the engine is spinning hard. As we said above, minimum vacuum occurs when the throttle is fully open.
Now, here’s the exciting bit: If the throttle is open, but engine vacuum is high, the engine is very likely to be operating with poor fuel efficiency! That’s in fact the reason that fuel efficiency is poor – the engine is working hard to drag air past the nearly closed throttle (in technical terms, ‘pumping losses’ are high).
In fact, arrowed here on the fuel consumption map is one example of where ‘high engine vacuum, throttle open’ can be located. As you can see, not very economical.
What we do with the FuelSmart is light a LED on the dash when the engine vacuum is above a preset level and the throttle is open. If the throttle is closed but vacuum is high (eg on engine over-run) the LED stays off, and if the throttle is open but the engine load is high (ie vacuum is low) then the LED still stays off.
So how do you use this dashboard indicator? It’s easy - if the LED is on, you should immediately assess your driving style.
For example, when going up through the gears, if the LED stays on, you should use more throttle between each gear change. If you are slowing for a set of traffic lights and the LED stays on, you should lift right off – not trail along with the throttle just open. In stop-start urban traffic and hilly areas (amongst the most difficult of conditions to get good fuel economy) you can use the LED indicator to guide you in variations of ‘pulse and glide’. (These conditions lend themselves well to pulse and glide because the throttle is naturally on/off a lot.)
In constant load cruise conditions, the LED will stay on – but that’s because in these conditions the engine is actually not working at its fuel-efficient optimum. If you are already in top gear, there’s nothing much more you can do. But if you’re not in top gear, the LED is a reminder to change up and then use more throttle to maintain speed.
Unlike many advertised fuel saving devices, there’s no shonky science behind FuelSmart. Instead it uses existing engine management sensors to alert the driver when they’re driving in a manner that’s likely to be using more fuel than really needed. By changing driving behaviour to keep the engine operating more often at better efficiency, fuel economy will be improved, especially in stop/start urban conditions and in hilly areas.
And that’s a fact!
Next week: fitting and calibrating FuelSmart
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