The current increase in gas prices and global warming makes improving fuel economy more and more interesting. Ofcourse, fuel economy is not the most sought after by motorheads, but still it's good to know how it works when you're just cruising. This way money is saved for more 'go faster' bits.

Besides the way the engine and car are built, the driver has a major influence on fuel consumption. How this works is easiest explained by the specific fuel consumption diagram. This diagram has the engine speed on the bottom axis and the torque or mean effective cilinder pressure on the left axis.
Figure one shows an example of such a diagram.



As can be seen, the torque curve is also plotted. This is customary, because it limits the diagrams top side. The brake specific fuel consumption (bsfc) is plotted using isolines (like in an isogonic map). Each line represents a certain bsfc.

What we can see is that at 3500 rpm and 4 bar mean effective pressure (mep), the specific fuel consumption is 300 grams per kilowatthour (g/kWh). At about 2200 rpm and 15.5 bar mep, the specific fuel consumption reaches its lowest point of 206 g/kWh.

One can clearly see that the bsfc drops when the load increases. This is because at higher loads, the air available in the cilinder is used more efficiently. Less air is pumped through the engine without being used for combustion. Another factor is that at high loads, the throttle valve will be in a more open position, causing less pumping losses. The pumping losses and pumping of unused air causes a lot of energy to be wasted, which shows in the high bsfc at low loads.

Knowing this, we can improve our own fuel efficiency. When accellerating, shift for example at 2500 rpm, and the engine will be in a lower rpm range, which means the load (torque) must be higher to put out the same amount of power. Because: Power = torque x rotational speed. The fuel will then be used more efficiently, because no unnecessary air is pumped by the engine, the throttle is not unnecessarily restrictive and there's more time available for combustion of the fuel. The downside is, the car will hardly move...

Difference between Otto and Diesel engine

Figure 2 shows the bsfc map of a diesel engine. The difference between this and an Otto engine is that the isoline of the lowest fuel consumption is open and reaches the torque curve. With an Otto engine, this isoline will be a little bit below the torque curve. Reason for this is that a Diesel engine always runs at a lean air-fuel ratio. All fuel injected will be able to combust. An Otto engine has its maximum power at a rich air-fuel ratio of 0.9, which means there's a bit of fuel injected that will not combust. This causes a rise of fuel consumption at full load.

The curves of constant power are also plotted in figure 2 (in blue). This is an exellent way to see that it benefits the bsfc to shift early. Lets say we're driving at the highway at 120 km/h and it takes 50 hp to overcome driving resistance. If for example in 4th gear the engine is running at 4000 rpm, the specific fuel consumption is 270 g/kWh. I we shift up to 5th gear and the engine would then run at 3000 rpm, the specific fuel consumption drops to 230 g/kWh. At the same speed!

The specific fuel consumption map has shown it's beneficial to shift early and drive at high loads to save fuel!