Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Interesting autospeed article on ram air intake

garys_1k · 06-28-2008, 05:14 PM

Quote:
Originally Posted by garys_1k
Reducing upstream restrictions just means you'll hold the throttle closed more, and increasing upstream restrictions means you'll hold the throttle open a bit more. The end result is the same.
No, thats not how it works. The engine will still breath the same ammount of air and have the same intake if you increase air pressure, reduce throttle angle and maintain the same load. Take the engine up to 30k feet where it can only produce 30% as much power at WOT and it will still consume the same amount of fuel at the same load.

Exactly, that demonstrates my point -- reducing restriction at one place (say, the air inlet system) and replacing it somewhere else (e.g. a more closed throttle valve, or high altitude) makes ZERO difference. We agree.

Originally Posted by garys_1k
Back in my max. HP days I played with that sort of ram air, but just like the equations say, there's pitifully little pressure available from forward speed until you exceed about 90 mph. In any case, for hypermiling, it's a waste of time.
Read the autospeed articles. Its not about pressure, its about reducing restriction. The large forward facing intake improves fuel economy by making the engine a more efficient air pump at low RPM and high load. This will allow you to use 1 or 2 gears higher at any given load.

Thanks, I did read them and am aware of how reducing pressure drop across the intake system can allow higher power via higher air mass flow. But when you're only asking for a relatively small fraction of the engine's peak torque (at its present speed) then those restrictions don't matter -- the total restriction, air system + throttle valve, do.

Getting good fuel economy is NOT about very high loads, it's about moderate loads at low-ish speeds. Lugging the holy hey out of an engine may seem to give great volumetric efficiency but the enrichment and spark retard will kill the overall efficiency. Most peak efficiency islands (BSFC) are about 2/3 or so of peak torque, maybe 3/4. Above that you get enrichment and spark retard, both FE killers.

The combustion chamber could care less what caused the particular mass of air to enter via the intake valve, throttle, dirty air filter or a piece of duct tape across the inlet pipe. All it "knows" is that there's a certain mass of air in there, plus a proportional amount of fuel, and when those are compressed they're burned and pressure is the result.

Low restriction air intakes are all about maximum torque. They won't let you upshift sooner at part pedal. In fact, given that pressure drop increases with velocity squared, they're biggest impact will be at high load at high engine speeds. Down in economy-RPM-land, like 1200-200 rpm, low restriction intakes are a total waste for fuel economy. Just keep the air filter reasonably clean and you're fine.

06-28-2008, 05:14 PM	#19 (permalink)
garys_1k EcoModding Lurker Join Date: May 2008 Location: Motor City Posts: 89 Thanks: 0 Thanked 1 Time in 1 Post	Quote: Originally Posted by garys_1k Reducing upstream restrictions just means you'll hold the throttle closed more, and increasing upstream restrictions means you'll hold the throttle open a bit more. The end result is the same. No, thats not how it works. The engine will still breath the same ammount of air and have the same intake if you increase air pressure, reduce throttle angle and maintain the same load. Take the engine up to 30k feet where it can only produce 30% as much power at WOT and it will still consume the same amount of fuel at the same load. Exactly, that demonstrates my point -- reducing restriction at one place (say, the air inlet system) and replacing it somewhere else (e.g. a more closed throttle valve, or high altitude) makes ZERO difference. We agree. Originally Posted by garys_1k Back in my max. HP days I played with that sort of ram air, but just like the equations say, there's pitifully little pressure available from forward speed until you exceed about 90 mph. In any case, for hypermiling, it's a waste of time. Read the autospeed articles. Its not about pressure, its about reducing restriction. The large forward facing intake improves fuel economy by making the engine a more efficient air pump at low RPM and high load. This will allow you to use 1 or 2 gears higher at any given load. Thanks, I did read them and am aware of how reducing pressure drop across the intake system can allow higher power via higher air mass flow. But when you're only asking for a relatively small fraction of the engine's peak torque (at its present speed) then those restrictions don't matter -- the total restriction, air system + throttle valve, do. Getting good fuel economy is NOT about very high loads, it's about moderate loads at low-ish speeds. Lugging the holy hey out of an engine may seem to give great volumetric efficiency but the enrichment and spark retard will kill the overall efficiency. Most peak efficiency islands (BSFC) are about 2/3 or so of peak torque, maybe 3/4. Above that you get enrichment and spark retard, both FE killers. The combustion chamber could care less what caused the particular mass of air to enter via the intake valve, throttle, dirty air filter or a piece of duct tape across the inlet pipe. All it "knows" is that there's a certain mass of air in there, plus a proportional amount of fuel, and when those are compressed they're burned and pressure is the result. Low restriction air intakes are all about maximum torque. They won't let you upshift sooner at part pedal. In fact, given that pressure drop increases with velocity squared, they're biggest impact will be at high load at high engine speeds. Down in economy-RPM-land, like 1200-200 rpm, low restriction intakes are a total waste for fuel economy. Just keep the air filter reasonably clean and you're fine.