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Fuel Economy -The Nerd Glossary

Jay Chen
Feb 20, 2009
Sccp_0902_01_z+fuel_economy_the_nerd_glossary+pic Photo 1/1   |   Fuel Economy -The Nerd Glossary

Engineers are strange. We think in the black and white and don't like to leave things undefined. In our little world, everything is a potential variable. That's why our writing is so boring and long winded. Ninety-percent of the time, we're just wasting time clarifying all the conditions so that a simple statement, like "I weigh 170-pounds" is absolutely clear. Just to be sure other nerds understand and my statement doesn't get shot down, I'd have to qualify my statement with where, the time of day and year, ambient conditions, and the calibration precision of the scale I just used, not to mention what planet I'm on. That's just how we think. But it can be a good thing because engineers, who are perpetually in search of efficiency, got smart and invented complex terms of their own to take out as many variables as possible.

It's the same when it comes to cars and why we like to use terms defined by overly complex equations and relationships. My hunch is that you're reading SCC instead of the average tuner magazine because you're either already a science nerd with an affinity for cars or aspiring to be one. So it's about time for you to start learning some of our nerd lingo.

MEP (Mean Effective Pressure)
Car know-nothings like to brag about how much power their machines can make. Outside of just unverified delusions, there's a lot of questions to be asked. How big is the engine, is it boosted, how much boost, what type of fuel, and how fast does it spin. Instead of talking in ambiguities and wasting time telling everyone what your engine is running on, MEP is a far better solution. We all know that torque is a function of how much pressure and force you can generate inside the cylinder. So why talk about power or torque when you can just brag about cylinder pressures (which doesn't care about displacement and encompasses compression ratio, gas, volumetric efficency and boost)? But bragging about peak cylinder pressures is like bragging about peak power on a big turbo car. What matters is how much pressure you can maintain, or the average/mean pressure acting on the piston. MEP is proportional to torque (which is a mathematic function of power) and gives you the bare bones story. Engineers often use the term BMEP (Brake Mean Effective Pressure), which also takes into consideration friction losses of the engine and describes just how good an engine is. BMEP for a naturally aspirated four stroke Spark Ignition (SI) engine at max torque ranges from 125 to 150 psi while turbocharged engines range from 180 to 250 psi. At peak power, BMEP is typically 10 to 15-percent lower.

SFC (Specific Fuel Consumption)
With everyone harping on fuel economy, they often forget that it's easy to make a dinky fuel efficient motor that uses no gas, but is also completely gutless. That's where SFC comes in. SFC is the ratio of fuel consumed divided by the power that an engine makes. It's ultimately a measure of efficiency in converting chemical energy in fuel into useable work and power. The lower the SFC the better since less fuel is consumed to produce more power. Forget MPG, we should all be talking about SFC, which at best is 0.47 lbm/hp*h on a SI engine. Compresion Ignition (CI) engines do better with SFC values of 0.32, which is why trucks use diesel, like all of us should be! By throwing the term brake in front of it, BSFC then becomes the measure of fuel consumption versus power production that also takes into consideration the drag generated by the engine itself.

Closed loop/Open loop
Fuel injection systems are always trying to deliver just the right amount of fuel to the engine to achieve the best combustion, which comes from a stoichimetric air/fuel ratio. To do that, oxygen sensors in the exhaust constantly monitor the oxygen content of the combusted gasses or Exhaust Gas Oxygen (EGO). Too much oxygen means that not enough fuel is available, while too little oxygen means too much fuel. This constant check and balance (or feedback loop) of fuel injected and EGO is called closed loop operation. Most cars operate in closed loop at part throttle and low loads. At WOT, the ECU jumps out of closed loop operation to open loop, which just injects a predetermined amount of fuel for a typically rich air/fuel mixture. Newer cars, or stand alone ECUs, can potentially use Closed Loop management even at wide open throttle since they can use wide band oxygen sensors to target A/F mixtures, even at WOT, that older narrow band sensors can't measure.

Equivalence ratio
Instead of using air/fuel ratios (A/F), which are a ratio of the mass of air divided by the mass of fuel consumed by an engine, engineers like to take it a step further. By dividing the actual A/F ratio by the ideal stoichimetric A/F ratio, 14.7:1, you get the fuel/air equivalence ratio, signified by the greek symbol (phi). So now we know if a mixture is rich, is greater than 1.0. Lean is less than 1.0. Why all the trouble? Because engine nerds don't always deal with gasoline like we do. Other fuels mixes, like diesel, hydrogen, ethanol or even ethanol gasoline, have different stoichimetric A/F ratios than the 14.7:1 we're use to for gasoline. In fact, common California unleaded pump gas, which has 10-percent ethanol, or E-10, has a stoichimetric A/F ratio as low as 14.1:1. We typically don't mention this since this is well within the compensation ranges of most ECU closed loop operations, but it is relevant. Sometimes we like to confuse ourselves and also use a relative air/fuel ratio called (lambda), which is the inverse of , where lean is greater than 1.0 and rich is less than 1.0. That's why people often call wide band oxygen sensors lambda gauges.

We use this term a lot since there's no better word for it. Stoichiometery simply means the complete or balanced chemical reaction of two or more properly proportioned reactants into another form without any leftovers. Burning fuel abides by the same chemical laws, and if you work out the molar balance, conventional gasoline and all the components of air react completely with a mass ratio of 14.7 parts air to one part gasoline. Stoichimetric combustion isn't just some geek number. It's also the ideal ratio for the most efficient fuel consumption, power production, emissions reduction, and highest combustion temperatures.

By Jay Chen
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