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EFI Tuning Fundamentals: Brake Specific Fuel Consumption

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Brake Specific Fuel Consumption

04.17

00:00 - In this module I want to talk a little bit about how the engine converts the energy in fuel into power.
00:06 The internal combustion engine isn't actually very good at this.
00:09 And most of the energy from the fuel is converted into heat and sound.
00:13 With only a small percentage actually making it to the crankshaft.
00:17 Some engines however do a better job of this than others.
00:20 And it's possible to have two engines producing the same amount of power while consuming vastly different amounts of fuel.
00:27 Likewise, we can have two engines consuming the same amount of fuel while producing very different amounts of power.
00:33 How efficiently an engine can convert fuel into power is expressed by the brake specific fuel consumption, or BSFC for short.
00:42 The BSFC measures how many pounds of fuel are required to produce one horsepower for one hour.
00:49 BSFC is expressed by the equation shown here.
00:53 And in order to make this calculation we need to know how much fuel an engine is burning and how much power it's making.
01:00 In imperial units BSFC is expressed in pounds per horsepower per hour.
01:06 While in metric units it's grams per kilowatt per hour.
01:10 The BSFC is affected by a number of aspects including cylinder head design, camshaft design, engine cooling, and compression ratio to name a few.
01:20 As an example, an engine using forced induction may use additional fuel to help control combustion chamber temperature.
01:26 This extra fuel is effectively wasted and hence forced induction engines typically have a slightly higher BSFC than a naturally aspirated engine.
01:36 For a similar reason, air-cooled engines will also use more fuel to aid engine cooling and hence their BSFC will also suffer.
01:44 To give you some idea of the likely BSFC numbers you could expect for some typical engine designs.
01:50 A naturally aspirated engine using a carburetor would typically have a BSFC in the region of 0.48 to 0.55 While naturally aspirated engine using fuel injection is slightly more efficient with the BSFC around 0.45 to 0.50.
02:08 With forced induction engines the BSFC suffers and we might expect to see a BSFC of around 0.58 to 0.65 with a carb, or 0.55 to 0.60 with EFI.
02:22 While these numbers are a good guide, some engines are still significantly better or significantly worse than these averages.
02:29 Consider for example an air-cooled Porsche engine fitted with a turbo.
02:34 It's quite possible this sort of engine could see a BSFC of around 0.70 or even worse.
02:41 On the other end of the scale is Honda's very efficient K20A engine which has a BSFC of approximately 0.3.
02:49 I like to think of BSFC as describing the amount of fuel that's wasted.
02:55 In this way, a higher BSFC number means more fuel is being wasted and less is being used to generate engine power.
03:02 A low BSFC number on the other hand describes an engine that wastes much less fuel and is more efficient at turning the energy in the fuel into power at the crankshaft.
03:13 As a tuner we probably aren't going to have a lot of influence over the design of the engine we're tuning.
03:19 So you may be wondering why we need to be concerned with BSFC at all.
03:23 While we may not have a lot of control over the engine's BSFC, it will have a big effect on the amount of fuel we need to supply.
03:32 This is important information to know when we're sizing the fuel system.
03:35 The requirements for both the injector and the fuel pump capacity will be affected by the BSFC of our engine.
03:42 Unfortunately we're also unlikely to be in a position where we can accurately measure the fuel consumption of the engine.
03:49 And hence calculate the actual BSFC for the engine we're tuning.
03:54 Unless we have this data.
03:55 You're going to be limited to using the approximations I've just mentioned.
03:59 If you're relying on this data for calculating injector or fuel pump requirements, I always tend to be conservative for the BSFC number and err on the high side.
04:08 This gives some headroom.