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Brake Specific Fuel Consumption

A really useful concept

by Julian Edgar

Click on pics to view larger images


This article was first published in 2008.

Do a web search under ‘BSFC’ and you’ll find results including Birkenhead Sixth Form College, the Bouncing Souls Fan Club and the Black Swan Folk Club. But my favourite is the Boston Society of Film Critics. I mean what do they do - sit around together, criticising films?

However, you’ll also find under BSFC this term – Brake Specific Fuel Consumption. And that’s what we’re going to talk about here.

Specific Fuel Consumption

When we use the word ‘specific’ in an engineering context, we’re describing something in the light of a comparison. So, ‘specific power’ is probably better known to you as ‘kilowatts per litre’, or ‘hp per cubic inch’. Simply, it’s the peak power divided by the engine capacity.

Specific fuel consumption is similar. It’s the amount of fuel consumed, divided by the power being produced.

So it could be expressed in litres of fuel divided by the kilowatts developed. In fact, the fuel quantity is normally described as a weight in grams or pounds, so Specific Fuel Consumption is expressed as:

  • Grams (per minute) per kilowatt (hour) - or just grams/kilowatt hour (gm/kWH)

  • Pounds (per hour) per horsepower (hour) – or just pounds/horsepower (lb/hp)

Sometimes you’ll find other units being used as well. However, don’t get too hung up on the units – remember, they all express fuel divided by power.

Brake Specific Fuel Consumption

So what’s Brake Specific Fuel Consumption? As with bhp (brake horsepower), this refers to the specific fuel consumption when the power is measured by an external brake – in other words, a dyno. Most times, 'brake specific fuel consumption' and 'specific fuel consumption' are the same thing.

Power, Torque and BSFC

The easiest way of understanding Specific Fuel Consumption is to look at an engine performance graph.

Click for larger image

Here’s a good one from a classic engine – the Jaguar V12 HE, the ‘HE’ indicating the use of high swirl heads.

The red line shows power – all the way to about 220kW in this graph. The green line shows torque (although here it is expressed as Brake Mean Effective Pressure). And then we have the newie – SFC, shown by the purple line. As can be seen, the SFC curve doesn’t initially appear as you might have imagined it would.

Firstly, at idle it’s about 280 g/kWh, then as revs rise, it drops to be at its lowest at about 2500 rpm (at say 270 g/kWh). From there it rises steeply to reach 350 g/kWh at 6000 rpm.

Firstly, why should the SFC be lowest at middle revs? Or, to put this another way, what causes an increase in fuel used per kW at both low and high revs?

At low revs, SFC suffers because there’s increased time for the heat of combustion to escape through the walls of the cylinders and so not do useful work. At higher engine speeds, the frictional loses of the engine rise alarmingly (especially in this case with 12 cylinders!) and so the energy of combustion is again being wasted, this time in heating the oil.

There’s another reason that SFC is lowest at ‘middle’ rpm. Because the engine is tuned to develop best cylinder filling (ie to produce best torque) at middle revs, the engine’s breathing is at highest efficiency at these speeds. But don’t fall into the trap of saying that SFC is always at its best at peak torque – that’s not usually the case.

But the real trouble with diagrams like the one above is that in many ways, they’re irrelevant to real-world fuel consumption. Why? Because these graphs are drawn for full throttle! So if the engine is powering a racing ski-boat travelling constantly at full load, then yes, the shown SFC data is all well and good. But what happens at part throttle, as occurs in nearly all normal car use?

Well, then, the situation is very different! And the trouble is, the SFC figures are always much worse...

If you’re having difficulties coming to grips with this (“What? He reckons fuel consumption gets worse at small throttle openings?”), remember that we’re talking about specific fuel consumption – the amount of fuel used per power developed.

And if the BSFC gets worse at lower throttle angles, the power must be being reduced at a quicker rate than the decrease in fuel consumption...

Light Load SFC

Click for larger image

This graph shows what happens at lighter engine loads – it’s from a Repco manual for “a typical four cylinder” engine. The SFC is expressed this time in pounds per horsepower hour – but as we said earlier, it doesn’t matter what units are used.

At 100 percent load (ie wide open throttle) this engine has a minimum SFC of 0.43 – see the bottom curve. As we by now expect, at both lower and higher revs that this, the SFC rises.

But have a look at what happens at 50 per cent load! The SFC results at half load and 1000 rpm (ie idle) doesn’t matter much (when would you be in that situation?) but at 2000 rpm, the SFC has gone up by 13 per cent. At 4000 rpm, it’s gone up by just under 30 per cent!

And keep in mind that in normal use, even 50 per cent is a lot of throttle. A more frequently used load is 25 per cent. At 25 per cent load, the SFC at 2000 rpm has risen by a massive 117 per cent over that achieved at full load! You can also see from the shape of the 25 per cent load curve, BSFC is even more heavily influenced than ever by the rpm being used.

So what accounts for this terrible decrease in SFC at just the throttle openings the engine will be used at most often? ‘Throttle’ is the key word here – as the engine is increasingly throttled, it has to work harder and harder at drawing air past the throttle blade. This is the reason that there is a measurable vacuum after the throttle blade – the engine is trying to drag in more air than it is being permitted to. Each time a piston is descending on the intake stroke, it’s having to do this extra work. Working internally hard as a vacuum pump means there’s less power available at the flywheel...

This work against the throttle restriction is referred to as ‘pumping losses’.

When You Close the Throttle

Click for larger image

Remember above where we said that if the SFC gets worse at lower throttle angles, the power must be being reduced at a quicker rate than the decrease in fuel consumption?

These graphs clearly show the effect. They’re taken from the 1976 edition of Oldhams New Motor Manual.

The top graph shows the power output at quarter throttle, half throttle and full throttle. The bottom graph shows what happens to the SFC at these different throttle positions – and doesn’t the quarter-throttle SFC rocket, especially at higher revs!

Real World

So if SFC is much worse at lower loads, what actually happens in the real world? Let’s start off by showing a SFC graph in a slightly different way.

Click for larger image

This diagram has engine revs along the bottom axis and engine load (expressed in BMEP) on the vertical axis. The lines on the diagram join points of equal SFC. This and the following two diagrams are sourced from a 1999 report prepared for the Canadian government by Sierra Research of California. The diagrams are based on a sample of 1995 model year, naturally aspirated, EFI 2-valve engines.

Click for larger image

Here colour has been added to the graph (good, isn’t it?!) to show more clearly where the different ‘islands’ are located. The best BSFC is the red area centred around 2000 rpm and three-quarters load.

Click for larger image

And now we have the killer. Here each dot shows the speed and load for a typical mid size car at 1 second intervals during the US fuel economy test. Of the time the car takes to do the test, just 5 seconds are in the island of best BSFC. Quite a few of the dots (the authors say that they overlay) are at worst BSFC – idling at zero load with the car stationary!

The full report can be seen at www.tc.gc.ca. It has some more interesting BSFC ‘island’ diagrams showing the advantages of turbo and multivalve engines.

More Diagrams!

Here are some interesting BSFC charts to look at.

Click for larger image

The Toyota Prius petrol electric hybrid uses its pseudo-CVT and strong low-rpm electric assist to keep the 4 cylinder internal combustion engine working as much as possible in the area of low SFC (red line).

Click for larger image

The Honda Insight hybrid uses a 3 cylinder VTEC engine. This diagram, taken from a French engineering investigation of the car, shows measured SFC for the engine. The testing was of the CVT transmission Insight and was done on a chassis dyno.

Note how the blue/yellow island of best SFC is achieved at relative high revs and load, and how there’s a second area of low SFC at about half load and 1500-2000 rpm. I assume that this second area is achieved through the VTEC mechanism, that in this car, at low revs shuts off one of the two inlet valves for each cylinder, promoting better swirl.

(Apparently the CVT Insight does not use lean cruise – that is, very lean air/fuel ratios at constant throttle. The BSFC map for such a car would make very interesting viewing!)

Click for larger image

Not every engine has its lowest BSFC at moderate rpm. This quad rotor rotary Le Mans engine has minimum BSFC at 6000 rpm!

Conclusion

BSFC is a good concept to have in your mind, not because it sounds impressive when you chuck it into a conversation about fuel economy, but because it makes you think about things in a different way.

For example, anything that allows you to keep the throttle open wider and the revs lower (like changing up to a tall gear and then holding it) will reduce fuel consumption because BSFC will be improved. But equally, recycling exhaust gas (ie EGR) might also achieve that same effect because pumping losses will be reduced - not all the inlet charge needing to come past the throttle.

Diesels, which we’ve not mentioned here, are much more efficient at low loads because they don’t have a throttle restriction in any type of driving – low loads are catered for by just reducing the fuel that’s injected.

As you can see, BSFC – and how it changes with load - is one of those fundamental concepts that helps explain a lot.

References

Austin, TC, et al, (1999) Alternative and Future Technologies for Reducing Greenhouse Gas Emissions from Road Vehicles, Sierra Research Inc

Bosch, (2004) Gasoline Engine Management

Holmes, L (Ed), (1976) Oldhams New Motor Manual, Hamlyn Publishing Group

Repco, (1972) Repco Engine Service Manual

Stone, R, (1985) Introduction to Internal Combustion Engines, Macmillan

Trigui, R, et al, (2003) Hybrid Light Duty Vehicles Evaluation Program, International Journal of Automotive Technology, Vol 4, No 2

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