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.
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