Intercooler Bypass

[Fix_Until_Broke]

Has anyone thought of putting a bypass valve on the intercooler?

Here's my thoughts - For most driving there is way more air than required for the ammount of fuel injected and since the compression ratio is fixed, the more air in the cylinder, the more work it takes to get the piston to TDC. Ideally there would be only slightly more air than required for complete combustion of the fuel injected. So, with the intercooler, we heat the air with the turbo (increasing pressure/temperature), then we cool that air with the intercooler, then we bring that same air into the cylinder just to compress it to heat it up to combust the fuel we inject. So for crusing conditions why heat up the air just to cool it, just to re-heat it again? In a maximum effort (or even moderate effort) we want as much air in the cylinder as we can get and the intercooler makes sense. But for crusing conditions bringing in only as much air as needed for complete combustion and having that air at even pre-heated by the exhaust would minimize the ammount of work that the engine does that never makes it to the wheels.

So...if one could bypass the intercooler during cruise conditions and not bypass it during high load conditions, maybe one could increase the thermodynamic efficiency of the operation.

Maybe it's not worth the hassle as the air from the turbo during cruise conditions may not that hot, but given the above (assuming it's valid) there is no reason to cool it either.

Any thoughts - just throwing this out there

 

[DrSmile]

The problem I see with this is that air will take the path of least resistance, so the less restrictive bypass would have to be sealed off pretty perfectly. You should be able to fabricate a valve that's activated as soon as boost is produced by the turbocharger. As for benefit, the only way I see it helping would be if the bypass allows the IC to cool off after heatsoaking. Probably not worth the effort, especially in a diesel. But heck it sounds like a fun project!

 

[jck66]

I read your post and wanted to make a few comments.

1. In a diesel, there is almost always more air in the charge that stoichiometry would dictate.

2. Under cruising conditions, your engine typically isn't making a heck of a lot of boost. You're only talking a few psi on level ground. And don't forget that fuel makes boost. If the car isn't injecting much fuel, it won't be making much boost.

3. If the air coming off the turbo isn't that hot (because it's not being compressed much), then the intercooler isn't cooling it much, either. In this case the intercooler represents a wide spot in the intake line, not much more.

I guess my conclusion is that the IC really only helps you. Kind of like your turbocharger, it doesn't get in the way when it's not needed.

 

[Figster]

The energy that goes into compressing the air is pretty much extracted when it expands on the power stroke. You get most of the energy back that is required for compression whether the engine burns fuel or not. Think of the compression/expansion event as an air spring...

Therefore, if you bypassed the intercooler, not a whole lot would be gained. You would probably get higher exhaust gas temps, which would be a little more energy for the turbo, but as was is said above, boost and therefore intake heat isn't really made unless the engine is under load.

As for running more excess air than stoichiometry, this is required due to the incomplete mixing of the fuel/air when it is injected. It is not possible (currently) to use all of the air that the engine ingests due to the injection/air mixing dynamics. This is why the injection pressures keep creeping upward. More injection pressure = better mixing.

The holy grail of engines is the HCCI, or homogeneous charge compression ignition engine, which makes better use of the air that goes into the cylinder. Basically, the fuel is premixed with the air before it enters the cylinder (like a gasser), but it is ignited by compression alone. With this engine, you get the good efficiency of the diesel (no pumping loss) with the low emissions of the gasser. The problem is that ignition timing is not easily controlled. There is currently lots of research into this problem...


Figster

 

[Fix_Until_Broke]

I know what you are saying regarding the control of ignition timing with the homogeneous charge idea - I tried propane in my 5.7 diesel (23.5:1 compression ratio) and it had a mind of it's own regarding ignition timing - maybe at 18:1 the propane would have not self ignited and would have lit with the fuel injection event)

Regarding getting the energy back on the power stroke, maybe if you assume isothermal conditions (or is it adiabatic?) - the one where you assume there is no transfer of energy across the boundries of the system (heat transfer from the cyclinder walls to the air or vice versa). I don't believe that this would produce a 10% gain in mileage/efficiency. I'm looking for a percent here/there that VW probably elected not to go after due to complexity/cost/market demands etc. I'm willing to sacrifice a little performance to get a little efficiency.

The increase in injection pressures I believe is for better atomization of the fuel and faster flame front travel across the cycliner. More smaller droplets are burning faster insted of fewer larger droplets burning slower. The mixing I believe is generally dominated by the combustion chamber and piston dome shape hence the dish and pointed center in the pistons to create swirl/turbulance during the compression stroke (remember prechambers?).

As long as we are achieving complete combustion (ie: no smoke) and sufficient air temperatures when the fuel is injected to ignite it, I don't see the benefit of having more air in the cylinder than required to achieve the above conditions. The question really is, is there a detriment to having more air in the cylinder than required to meet the above conditions? If there is absolutely no detriment then end of discussion, but if there is then there is potential. After that the decision needs to be made as to if it is worth going after or not.

 

[03_01_TDI]

Doesn't the Lupo and some of the Polo come without a intercooler?

 

[moondawg]

As long as we are achieving complete combustion (ie: no smoke) and sufficient air temperatures when the fuel is injected to ignite it, I don't see the benefit of having more air in the cylinder than required to achieve the above conditions. The question really is, is there a detriment to having more air in the cylinder than required to meet the above conditions? If there is absolutely no detriment then end of discussion, but if there is then there is potential. After that the decision needs to be made as to if it is worth going after or not.

Less air into the cylinder means less mass taking part in combustion. This will mean higher EGT's.

Not passing the charge air through the intercooler will also raise EGT. (EGTs increase 1 degree for every 1 degree increase in charge temperature)

Less air into the cylinder will also reduce cylinder pressures, reducing combustion efficiency.

Of course, since you're operating at lower loads anyway, these effects may not be observable.

Summary: There is a net BENEFIT of having excess air taking part in diesel combustion, unless for some reason you're expending alot of energy to get it there.


moondawg

 

[Fix_Until_Broke]

Less air into the cylinder will also reduce cylinder pressures, reducing combustion efficiency.

I will need to go back and understand why reduced cylinder pressures reduce combustion efficiency. I recall that higher compression ratios incereased thermodynamic efficiency, but also don't remember why. Time for me to do some reading tonight.

Are higher EGT's a problem at normal crusing operation? What is the implication here.

Thanks for the insight/feedback

 

[moondawg]

Less air into the cylinder will also reduce cylinder pressures, reducing combustion efficiency.

I will need to go back and understand why reduced cylinder pressures reduce combustion efficiency. I recall that higher compression ratios incereased thermodynamic efficiency, but also don't remember why. Time for me to do some reading tonight.

Are higher EGT's a problem at normal crusing operation? What is the implication here.

Thanks for the insight/feedback

No, EGT likely wouldn't be a problem... they'd just be higher. The only detriment would be slightly higher NOx.

moondawg

 

[Ernie Rogers]

Hi, Figster,

Your post tells me you know a lot about cars. It's embarrassing to confess, I really don't know much about cars. I have a computer program I cheat with. I ran an engine simulation at two different intake temperatures, 333 deg. K and 363 deg. K, but held the fuel injected the same. The turbo boost for both was +3.7 psi. Well, I got the same power and efficiency. However, my model doesn't include combustion differences, and the case with higher input temperature had the compressed air temperature (before injection) running about 100 deg. K higher.

Also, the two cases had different A/F ratios, because the air mass was less for the higher input temperature. This only is an important factor if you are near max power-- no intercooler means less air in, and therefore less max power.

About the "holy grail," HCCI, well, nobody ever found the holy grail-- I think your label for HCCI is very apt, probably just money wasted. And, if they succeed, it still won't be as efficient as my TDI engine.

Ernie Rogers

The holy grail of engines is the HCCI, or homogeneous charge compression ignition engine, which makes better use of the air that goes into the cylinder. Basically, the fuel is premixed with the air before it enters the cylinder (like a gasser), but it is ignited by compression alone. With this engine, you get the good efficiency of the diesel (no pumping loss) with the low emissions of the gasser. The problem is that ignition timing is not easily controlled. There is currently lots of research into this problem...

Figster

 

[karlaudi]

The turbocharger (turbo) is designed to provide forced induction of the intake air without the need to use engine power to drive the turbine. It is, if you will, a "free" engine power-increasing device.

But because we are compressing the intake air it gets hot, which is not what our engine wants. What it wants is cold dense air and as much as it can get. So we use an air-to-air radiator, or intercooler, to cool the intake air charge downstream from the turbocharger. Remember the air for all intents and purposes is still "charged "or compressed air.

In Volkswagen's new 1.4 TSI motor, the intercooler reduces the intake air charge from about 200 degrees centigrade to just 10 degrees C. * (* Volkswagen Driver Magazine, October, 2005, page 16)

A diesel engine always runs at wide-open throttle. There is after all no throttle plate and the fuel delivery process controls engine speed and power.

With forced induction, two things happen. Because we introduce more air, we introduce more oxygen, which aids in a more complete burn of the fuel, but we are also increasing the volume and mass of the air now in the combustion chamber well beyond what the engine can breathe in on its own.

Lets say the turbo can double the swept volume of an engine with a CR of 22:1. Our engine, which has a swept volume of 1 liter in non-turbo form compressed 22 times smaller on the compression stroke, will then have 2 liters equally compressed 22 times.

Since we still have the same fixed volume whether the cylinder is at top or the bottom of its stroke, but double the volume of air, we have effectively raised the static pressure in the cylinder. We have essentially increased the amount of force that wants to push that piston back down to the bottom of its stroke too. After all, an internal combustion engine is just an air pump and therefore the more air you can get in on the intake stroke the more air pressure you can get out.

But we don't let the, now doubled, volume of air with its higher pressure out right way. We heat it again as we compress it and, as we do that more or less, we use the increasing thermal energy to ignite fuel, which then raises the pressures even higher, expanding with greater force in the same very confined space. This now greatly increased force is pushing down on the same amount of area at the top of the piston, resulting in a greater force applied to crankshaft and resulting in an engine that produces more power and torque than its normally aspirated version.

Since we have in effect increased the volume of gas going inside the cylinder we have also increased the amount leaving the cylinder which provides more "energy" to drive the turbo which increases more intake air which, in turn, increases more exhaust gases until the engine is working so hard, it, in a sense, explodes. This is why turbo engines have waste gates to limit boost.

Volkswagen, as with other manufactures, does not, and has not, always used an intercooler with its turbo diesel engines. I will have to check my archives to give an exact example by Volkswagen, or others, to know for sure, which turbo engine had an available, optional, intercooler to produce more power than its non-intercooler version.

As I recall, it was in the mid-late nineteen-eighties that VW offered an Eco-diesel engine option for its home market, and select European markets, in which the turbo was fitted not to raise power, but to reduce emissions by raising the amount of oxygen available in the cylinders for combustion yet producing nearly the same amount of power as then current normally aspirated Golf Diesel model.

So, I believe by by-passing, the turbo's intercooler you will produce less power overall, while eliminating that surge of torque that make passing in the 50 to 70mph range so easy and satisfying.

 

[karlaudi]

Speaking of efficiency, This German Company may be of interest.



ELSBETT Technologie GmbH
Weissenburger Strasse 15
91177 Thalmaessing
phone: + 49 (0) 9173 - 77940
FAX: + 49 (0) 9173 - 77942
info@elsbett.com


http://www.elsbett.de

 

[jck66]

In Volkswagen's new 1.4 TSI motor, the intercooler reduces the intake air charge from about 200 degrees centigrade to just 10 degrees C. * (* Volkswagen Driver Magazine, October, 2005, page 16)

Maybe on a sub-freezing day. You're never going to cool the intake charge to sub-ambient.


And in response to a question by fix-until-broke above, "isothermal" means at a constant temperature and "adiabatic" means zero heat transfer out of the system.

 

[AndyBees]

Based on your rationale (and I don't disagree), then a clogged (on the outside) Intercooler could/would lead to a loss in performance...... hence, a loss in fuel economy!

 

[lucki]

Egr

I think EGR put hotter air (without oxigen) from exhaust into engine to reduce NOx. EGR is open when you are demanding lower power. Perhaps reprogramating egr to open more time makes better mpg but less power. Perhaps adding hotter air with oxigen makes better mpg reducing air from egr and adding more power. Only empirically we get good results.

 

[otbBlaine]

So...if one could bypass the intercooler during cruise conditions and not bypass it during high load conditions, maybe one could increase the thermodynamic efficiency of the operation.

a very similar topic to this was discussed in another thread. I won't go into all the details, but basically you want the hottest cylinder conditions you can get to get the best thermodynamic efficiency. There are many interrelated factors to consider though, but an intercooler really is neccesary to keep the charge temperature low enough to keep the pistons from melting.

 

[Fix_Until_Broke]

a very similar topic to this was discussed in another thread. I won't go into all the details, but basically you want the hottest cylinder conditions you can get to get the best thermodynamic efficiency. There are many interrelated factors to consider though, but an intercooler really is neccesary to keep the charge temperature low enough to keep the pistons from melting.

yes, an intercooler is helpful in that regard, I will keep an eye on it with my EGT gauge. My post turbo EGT's are usually ~500-600F while cruising so I don't think running without an intercooler will put me in the danger zone. My peak EGT's were ~950 while pulling a 1700 lb trailer up a 6-8% grade at WOT for 2 miles. That would require the bypass to be semi automated (not too hard to do). But for experimentation a direct connection from turbo to intake would show what we want to see.

 

[woofie2]

Having a system that switched at around 6 PSI of boost would work, basically while cruising you are only pulling 4-7 PSI of boost, then at wot or higher load requirements you are pulling 15-25 PSI
It would also be good to install a fan on the intercooler so that it would be more efficient under under higher load.


Now then for a few questions-
Why does a larger FMIC result in better highway MPG for the same vehicle?
Why does an Audi TT fender liner result in more power and better highway MPG?

or is this just because the Computer dials back the timing for warmer Intake Air Temperatures?

 

[Fix_Until_Broke]

Having a system that switched at around 6 PSI of boost would work, basically while cruising you are only pulling 4-7 PSI of boost, then at wot or higher load requirements you are pulling 15-25 PSI
It would also be good to install a fan on the intercooler so that it would be more efficient under under higher load.


Now then for a few questions-
Why does a larger FMIC result in better highway MPG for the same vehicle?
Why does an Audi TT fender liner result in more power and better highway MPG?

or is this just because the Computer dials back the timing for warmer Intake Air Temperatures?

I was going to have it switch on intake air temperature (oddly enough ) They make thermopneumatic valves in a wide range of temps that could actuate somethig like the anti-shutter valve to open/close a bypass.

I have not seen evidance of a larger (or any) FMIC increasing MPG - or the TT fender liner doing the same. Not saying that this is not true, but have not seen any documentation.

Injection timing definately will effect efficiency. Could one check this by fooling the Air Temp Sensor (Kerma mod?). Unfortunately, coming into the winter season I can't gather much meaningful data due to the fuel/weather/driving changing variables on me.

I currently need to get a way to mount the MAP/IAT sensor somewhere in the intake tract before I can bypass the intercooler. Maybe I'll have to breakdown and get a Dieselgeek racepipe with that bung in it.

 

[Rick Bischoff]

Add-on question:
Could this be used to improve cold weather starts?

 

[Fix_Until_Broke]

Add-on question:
Could this be used to improve cold weather starts?

Short answer - No, it won't improve cold weather starts