More MPG for blocking off the air intake to the air compressor?

If it did matter, the only time would be when the air compressor is loaded and you are using full power of your engine. How often does that happen unless your one of those dweebs running around with a major air leak?

 

Probably make about as much difference as an AirCell intake insert. That is, not much, from what I've seen.

 

We had a lot of units years ago and the air dryers at the time had the purge valve that did not block the boosted air from passing thru. mostly 3406B
At that time the air compressors were blowing a lot of oil and we were changing them a lot. we tried installing a #12 hose into the air intake that involved welding a bung into the intake piping, the hose and fittings, it made the problem a lot worse because the compressor was continually under a vacuum, and it was actually pulling dirt into the compressor, because the purge valve did not block the air.
I bought a remote filter and a spacer plate, because we did not have a compressor to install, he was away for about a week and when he came back the compressor was within specs we changed it anyways because it was already approved.
The remote filter, spacer plate change was about $20.00 max at that time and doing the in the intake was around $500,00. and took a lot of time.
The drivers did say they got about 1/2 a gear better power and better jake performance, I believe that was from the air dryer being out of the air intake system.

 

even a minor leek will require the compressor to turn on everyone in a while.

What about the up hill after a long down hill? Air pressure might be a little low if you used brakes on downhill at all, but then your going full power up the hill. The compressor steals air from the motor at the most crucial time.

 

If it were 100% crap then you would never gain mpg/power after replacing air filter. If you starve the motor for air you do get less power/mpg. If your filter isn't brand new, the intake could be restricting the motor and the compressor would only add to the problem..

 

I would think that it you'd get better MPG by using intake air for the compressor. Think about it, there is more than enough extra air running through a diesel. Sure a restricted air filter could kill that extra, air leaks could also suck it up, but who in their right mind neglects those things AND cares about improving MPG? If all is well, there is plenty of extra air and if it is a wastegated turbo, it will just spin a little faster to make up for any boost loss anyway. The turbo is a far more efficient compressor than the air compressor. A turbo has an adiabatic efficiency of 75-80%, a piston compressor is closer to 30%. If the compressor loads at lets say 30 psi boost pressure, it is getting a 30 PSI inlet pressure increase, being a positive displacement compressor, it will compress to the same space, which mean a higher air density per stroke, without extra parasitic power losses, it will not have to run as long to fill your tanks.

Just how much more efficient is it? I stand by my 1st post, you'll get no measurable difference.

 

A lot of things have no measurable difference but still save fuel. The most accurate guys can maybe track +/-.3 mpg. Anything less then that could be variable in road, wind hills, load etc....and thus cant be measured accurately. Something that saves .1mpg cant be measured but will save 1000 on fuel per year. Wheel covers have no measurable difference per truck, but schnider knows they work do to fuel saving on their whole fleet.

Even without parasitic losses, you still could have used up your air braking on the down hill and need to recharge. If air filter is perfectly clean it might not make a difference, but in reality, many trucks struggle to get the air they need. rain will temporarily reduce air flow as the filter gets wet. adding a compressor into the mix certainly wont make it better. Removing it from the mix certainly wont make it worse. How many of you can leave your truck over the week-end and still have full air tanks?

Almost all trucks have some parasitic loss. Adjusting your seat, using your horn, taping the brakes etc can all use air. On some trucks and buses the wipers even use air. Lets not forget the adjustment of air ride suspension as you bounce over the road. Make a turn with a dump valve on and you need to air that axle back up. Go over a speed bump and the leveling valve will try to equalize the air and dump it back as soon as your flat. Turning power divider on uses air. Shifting even uses air. There are tons of parasitic uses of air even if your truck is in perfect condition.

 

I think you missed the point. The parasitic losses I was referring to were the air compressor itself, as in how much power it takes to compress the air. I never doubted that the air compressor could load while pulling a hill, I agree that it can and does.

Where I disagree with is that a compressor robs air from the engine. It's possible, only if there is an intake restriction, and a pretty big one that would drop boost pressure when the compressor loads. Changing that well overdue air filter(s) is the answer, not relocating the air compressor inlet.

Now, the air compressor is not a very efficient compressor and is directly driven, this is the parasitic power loss I'm talking about. The turbo is driven mostly by wasted energy and has more than twice the adiabatic efficiency. The reason most manufacturers use intake air is to take advantage of that. They claim about a 35% average increase in compressor adiabatic efficiency and more than 50% reduction in compressor on time when 30 psi is fed to it. Knowing that the air compressor is a positive displacement compressor, 3 times the intake density would make it pump 3 times faster on the same power. That's assuming a 100% adiabatic efficiency though, so it not 3 times as fast, but much better.

I have no proof of those claims, they are from the instructor at a Detroit class. I that same class, someone asked if it was better, why did they stop doing it? The answer was that EGR soot reduced compressor life, so they switched to a tube to the air filter. They were working on a baffle system to keep soot out, but gave up when they started using intake throttle valves to regen. That pulls a vacuum in the intake and hinders compressor efficiency.

So that is my opinion, making the compressor more efficient and running it less is better, even if the turbo has to spin a little faster to maintain boost.

If you want to crunch #'s on intake flow and engine demand, figure the CFM that the engine requires at 100% volumetric efficiency and what the intake system will flow (this varies with trucks). What you'll find is that the intake system generally flows more than twice what the engine requires. Now subtract 550 CFM for the compressor at maximum flow. You'll find that there is 1000-1500 extra CFM to allow for air filters clogging. Do air filter changes when needed and the compressor wont hurt a thing. If you truck has an air filter restriction gauge, there is no excuse for this. We change ours at 10 in/hg, which is 1/2 the restriction as what Pete recommends, but that's almost 2 years in our application and long enough.