Twincharging - calling Warpspeed! 3

[madmac666]

thread71-174690

Hi guys, i subscribed to this forum because i found the thread (referenced above) on twincharging while doing a Google search for information on this subject.

I have to say, the thread and contributions from forum users was very good and informative, which gave me a direction to follow for my own twincharger build.

Some background on this: I have a 1997 Mitsubishi Lancer Evolution 5 which i've used for sprints and hillclimbs in Scotland for a few years now. The state of tune of the car is such that we were producing around 650hp, but with the big turbo at 2.6bar, lag was always an issue. Twincharging seemed the most sensible option for the car to eliminate the lag and hopefully produce a torque curve with spool much lower in the rev range.

I'm using a Harrop HTV 1320 blower and the existing turbo which is a hybrid T04Z with 0.82 housing. This is the turbo that gave us 650hp previously, but on a 0,63 housing. The system is compound compression with the turbo feeding the blower. The blower has a 75mm pulley and the crank 150mm pulley so a ratio of 2:1.

I have the blower mounted and all pipework finished and finally had her on the rolling road a few days ago. We saw some impressive torque figures but had to cut the session short due to belt slip on the blower pulley when the turbo reached 1.5bar. The system i have uses a 32mm toothed belt but the length of the belt seems to be an issue as it's stretching and allowing it to jump over the pulley when the turbo spools up. We also had issues with controlling the turbo boost and decided i need to install another wastegate to allow better control of the boost. I'll have that done by next week RR session so we have control of the turbo boost and can carry on mapping.

It seems the belt starts to jump when the turbo reaches 1.5bar and the drive simply cannot cope with the power needed to increase boost by a factor of 2. I have a larger pulley (100mm) which i'll be fitting over the weekend and hope the longer wrap around of the belt on the larger pulley and the reduction of the drive ratio should sort out the issue of the drive. If it doesn't then i have another modification i can do to run a shorter belt on the blower and split the ancilliaries onto 2 separate belts but i'd like to try this again with the reduction in drive frst before i go to the trouble of a re-design of the belt drive. Space is at a premium so 32mm is the widest i can go and still have a chassis leg...

We never went above 4000rpm. Despite this, we saw 460ft/lbs torque and 360hp at 4000 and 2.9bar which confirms there's definatley power to be made! Obvously slowing down the blower will mean the turbo will need to be producing more to reach our goal of 2.5bar total. The outlet temp of 80c from the turbo at 1.5bar was logged, which is about the limit of the blower seals. I hope to keep the turbo around 1.5 bar so the outlet temps don't fry the blower seals. The combined air then goes through a very good intercooler and reduces the final temps to 16c (this is Scotland remember....!) at the plenum.

I think we're almost there and hope to have it mapped on the larger pulley with the extra wastegate fitted for turbo control. Looking at the RR graphs and data we collected the other day, we monitored the pressure from the turbo and combined total and can clearly see a multiply of the turbo by the blower very close to 2:1 from 1800rpm to 4000rpm but we couldn't control the turbo and it looked like it would have carried on climbing which would have killed the engine or the blower.

Because the engine already had the big turbo and supporting mods to make good power, we hope to see some good figures once the issues are ironed out. The engine itself is built to produce 1000hp 'reliably'.

I would like to hear any views and comments on my findings so far. I had considered swapping the turbo and blower order to reduce the power needed to drive the turbo but i felt the extra restriction on the exhaust (because the turbo would be working harder than it used to) would likely end up making less power, as the exhaust back pressure would then be much higher. It would certainly reduce the power needed by the blower


Thanks for posting the useful information, it really was a huge help and if you have anything to add that could help us, i'm all ears!

Donald

 

[madmac666]

Pat, That was also an option to cure the problem however due to the length of the pipework from the SC and plenum there would be a delay on backing off and also when the throttle is nailed. my concern obviously is taming the 800+hp while driving in anger and the last thing i need is for a delay in the engine power if the car starts losing traction mid corner!

I'm giving this some serious thought however!

 

[Warpspeed]

I agree, a throttle upstream of a blower only works if there is minimal trapped air volume between blower and intake ports. That completely rules out any kind of remote mounted front air to air intercooler with long pipework.
It can work pretty well on a Vee engine, and that is very fortunate, because there is not really any other choice.

The more air volume you have between throttle and intake valves, the worse the throttle response will be.
Especially when you close the throttle.
While that is not such a big issue with an automatic transmission, it can be a real obstacle to quickly shifting a manual box.

It also causes the supercharger to act like a vacuum pump at very small (high rpm) throttle openings, which can consume a surprising amount of crank power.

If you are running an EFI engine of any description, you would be well advised to leave the throttle in the original position, if at all possible.

The best way to completely unload a roots blower at light throttle, is with a direct opening air bypass around the blower. If you do this, a clutch is not required, and the effect is almost the same.

My own testing of various bypass systems, has repeatedly produced around a ten percent fuel economy improvement on a typically mildly driven road car. That is probably why every factory roots supercharged car that I know of, uses a blower bypass of some description.

A blower bypass is not required for serious off road competition, where light throttle fuel economy is of absolutely no concern.

 

[madmac666]

We tried disconnecting the bypass actuator from the plenum but there was so much pressure building up before the throttle butterfly it made it very heavy in use and caused massive heat build-up in the SC. The bypass i have is within the SC casing and operated by plenum vacuum. What we saw was essentially a huge volume of air at high pressure being forced through the small opening at light throttle. This reduced the vacuum in the plenum enough to start closing the bypass and increasing the pressure even more until the SC overheated.

I'm not sure if it would make a difference but next time i'm on the dyno i'll try swapping over the position of the actuator pipe into the plenum to the other end. It may be that where we have it connected close to the throttle butterfly that the air being pushed past the butterfly is creating a venturi effect within the plenum. Perhaps a long shot but worth a try for all the time it would take. There's fittings all over the manifold for various pressure/vacuum feeds so simple enough to move some of the pipes around.

Can i attach photos to the thread?

 

[Warpspeed]

What you need is an external air bypass plumbed in just ahead of the throttle body, back to the supercharger intake.

I can recommend an external turbocharger wastegate, fitted with a suitably light spring for this application. There is another thread here at Eng-Tips where I go into how to do this in some depth. The details are all in the last two posts of this thread :

http://www.eng-tips.com/viewthread.cfm?qid=174690

Give this a try, I think it will bring a smile to your face.

 

[madmac666]

I had a good read of those posts a while back and it makes perfect sense. The bypass fitted to the SC is essentially a butterfly valve that's controlled by an actuator that takes a vacuum signal from the plenum.

The actuator itself is double-ended, similar to an external wastegate with one outlet above and one below the diaphragm. I've only plumbed up the vacuum side but it has the port for pressure too. I spoke with the manufacturer who advised me the port for pressure is only there because they use a generic actuator which has this already. They didn't think i could plumb it in using both ports to any advantage. Thinking about it now, the bypass shaft has a small spring similar to the kind found on throttle butterflies so perhaps i could use the existing setup ?

The butterfly is mounted in such a way it would close on pressure out of the SC even if the spring and actuator failed.

I'm guessing there would be a way to plumb this up using both vacuum and pressure and pretty much do the job of a wastegate but without the extra plumbing work? After all, it's very similar in function and being internal to the SC is also out of sight and keeps it simple.

This would be my preferred route for the moment so any suggestions on making this work would be welcome

 

[Warpspeed]

Eaton produce several variants of their bypass system to suit various OEM requirements, so not sure exactly what you have there.
Earlier it was mentioned that a larger bypass was being fitted while the blower was being rebuilt, so I assume the whole bypass assembly is external to the actual blower casing itself.

If the pressure intake to the bypass can be plumbed after the intercooler, instead of right at the blower discharge, it should solve the temperature buildup problem.

I suggest you try using both ports on the actuator, and plumb it directly across the throttle body. Success will depend on the relationship between diaphragm area and the internal spring.

I have in the past encountered difficulties using butterfly throttles as air bypasses. The problem is the grossly non linear relationship between opening angle and flow area. They can also be prone to flutter under some conditions. A poppet valve is far more linear in flow versus actuation pressure, and they work just as smoothly in the supercharger air bypass role, as a more normal turbo exhaust turbine exhaust bypass.

Anyhow, it is all very well to speculate, but trying out a few ideas on the road under actual driving conditions will quickly tell you how drivable the beast is, when transitioning from small throttle high vacuum to full throttle max boost (in both directions). When done right it will be absolutely seamless, and drive like a factory car.

 

[madmac666]

The bypass is machined into the casing on this SC and the butterfly is essentially a valve between the inlet and outlet ports so it's all internal. I still haven't had an answer on whether or not the bigger bypass valve can be fitted so it's maybe a non-starter.

I think i'll try the actuator ports on each side of the throttle and see how it goes. I can always fit an adjustable restrictor in each line to the actuator to slow the bypass valve movement to fine tune it. The manufacturer edvises me they have seen this done with great success I seem to remember a fundamental reason why using both ports wouldn't work but i can't think what it was now....!

I'll be speaking to the manufacturer again tonight as it's the only time i can get hold of them due to the time difference

 

[madmac666]

Update:

Just a quick update. The car has been running fine as it is, although the blower is damaged and hasn't been replaced yet due to not having coin in the bank. I've done another 4 events since i last posted and have to say the setup is proving reliable in daily use. We've identified the reason for the blower overheating and added a BOV to act as a safeguard. The ratio between the turbo and blower pressures has altered slightly due to the blower slowly dying but the combined boost remains the same. This only gives a problem with turbo outlet temps for the short term. I built a tank to hold iced water and can pump that around the chargecooler for the short runs we do on the hills. This copes quite well and we're experimenting with anti-freeze/water at -18c from the freezer to run the chargecooler. So far so good.

Also just installed a temperature monitering system to keep an eye on 6 different temps throughout the system. I haven't yet used the car since installation so i can't comment on actual temps until after this weekend, which is my next event.

So far though the car has set aother 2 records and taken it's second FTD (fastest time of the day) since i last posted on here! Once we get a new blower back on, fit a bigger exhaust housing on the turbo and a larger throttle body, we should see some real horsepower. The map that's been done is also only a safe map and overfuelling so more power to be found there once the final bits are fitted and it's mapped for power

Will let you know how it goes

 

[bigjuicesr20]

Do you have a website or any more videos of your Evo?

 

[madmac666]

All the videos i have are here:

http://www.youtube.com/user/evomadmac

- enjoy, i enjoyed making them!

 

[gruntguru]

I have always wondered about a system with the turbo between the blower and the engine. Sure the speed of the blower pumping atmospheric air will establish the upper limit for airflow, but three advantages of this arrangement are tempting.
1. Because the blower limits the top end airflow, it can be geared to deliver much higher boost at low rpm than the alternative setup.
2. At the top end, the blower will be adding power to the crankshaft courtesy of the low pressure between the blower and the turbo. This power is sourced from the exhaust turbine so efficiency will benefit on this count as well as the higher compression efficiency compared to a roots operating at high boost.
3. Boost control is much simpler although the turbo pressure ratio would need to be monitored during initial setup, to avoid overspeed.

On another note. There have been several mentions of using a valve to bypass the blower at high boost levels. Of course this will flow backwards when open although some posters seem to indicate it would flow toward the engine. What's important of course, is that it would unload the blower by equalising the pressure above and below. It would probably also eliminate the need for the second wastegate by increasing the PR and therefore the load of the turbo compressor.

 

[gruntguru]

Also noticed reading the older thread linked by Warpspeed in the original post, some questions about mapping injectors located both pre and post blower eg Patprimmer "How you map for EFI with injectors into 2 different pressure zones is a problem. Maybe 2 different ECUs. a basic primary unit to put some fuel into the blower and a more sophisticated unit to accurately trim fuel for correct cyl to cyl distribution. Sounds expensive and complicated to me".
The simplest way is to run two fuel pressures, each pressure referenced to the intake pressure of the space they are spraying into eg: fuel flows to higher pressure injectors first (post-blower) then pressure reg #1 (with reference hose connected to manifold post-blower), then to rail #2 feeding injectors pre-blower, then pressure reg #2 (with reference hose connected to manifold post-blower). The two regs could have the same pressure setting if desired, the actual fuel pressures will differ by whatever the pressure increase across the blower happens to be.

 

[madmac666]

All part of the debate and good to hear alternative views on compound charging.

I settled on building mine the way i did to keep the exhaust pressure lower than the intake to allow a fuller cylinder charge, after all, that's where the power is made. Running a turbo after the blower will result in high exhaust manifold pressures as the turbo has to work harder. For the kind of power i'm wanting, i have to bear in mind the exhaust valve temps so filling the cylinders more with fresh charge will help with valve cooling and reliability of the engine.

I guess preference on plumbing will depend on the final use of the car. In my case, i just want torque and power - and fuel consumption isn't a consideration!

Modern fuel injectors are able to flow higher rates than only a few years ago and still keep a decent spray pattern at idle so twin injector setups shouldn't be needed (on my build anyway). On V-Power fuel, we're getting away with 1600cc injectors in their original position - so far we're only at 75% duty cycle although i expect that to rise once we get round to doing the final changes. I may yet start using E85 and would expect to then need 2200cc injectors

 

[Warpspeed]

Have to agree with MadMac, the very wide mid range power of twincharging comes from keeping induction pressure sufficiently above total exhaust back pressure, and the excellent scavenging and improved combustion stability that provides.

An added bonus is the high top end airflow that only a centrifugal type of compressor can provide. It all adds up to a wonderfully drivable package, with excellent reliability, high detonation resistance, and long engine life. In other words, power that lasts.

 

[gruntguru]

Sounds a convincing argument to me. The same applies to blower bypass. This would also eliminate that "positive pressure differential across the engine" advantage.

 

[madmac666]

Update:

I've replaced the damaged SC and been back to the dyno. This is the first time the system has been in good health since it was first put together so as far as figures go, we didn't know how it would perform - or stay together

Result was just over 700ft/lbs torque at 5500rpm, no lag at all and a very healthy spread of torque from 2500rpm up to 7800rpm. Power at the flywheel estimated at 700hp. Estimated because the dyno only gave wheel figures and wasn't setup for coast-down losses. Going by the previous visits to a different dyno, we have estimated 100+ hp and torque losses through the 4wd system.

It's incredible to drive now. From 2000rpm it pulls hard even in 5th and picks up a fair pace very quickly! Overall i think a very worthwhile modification as the car is very very quick on the hillclimb tracks. No waiting for the turbo to spool on the exit of the hairpins, it's instant torque at any revs and speed!

The last few events i've suffered clitch slip so a triple plate Superclutch has been fitted. There's also been the odd transmission breakage due to fatigue. I stripped first gear on the line and then twisted the centre diff output shaft at Doune. Both breakeages have evidence of classic fatigue and had reached the end of their life.

Here's an in-car run of Doune Hillclimb with 700ft/lbs torque:

http://www.youtube.com/watch?v=uGyLHvqifgU

I'm not having to rev the engine above 6500. The shift lights are set to start at 5500rpm although i was learning the car all over again. It's not been the same at each event!

I've attached the torque graph but not sure if it will show up on the thread. The dyno software had trouble matching speed to revs on the graph but the dyno operator assures me the readings are correct, just the scale has been affected for some reason

Happy to say Twincharging the engine has been a success!

 

[Warpspeed]

Hehehehe, another convert to twincharging that will never again be entirely happy driving with either "just" a turbo, or "just" a supercharger.

Those are some pretty impressive figures, from what is in reality a rather small capacity engine by most standards.

Unless you have ever had the personal experience of driving something like this, you would never believe how smoothly and willingly it pulls at low rpm in the higher higher gears around town.

And unlike many other highly tuned very high output engines, it will still be going strong for many many tens of thousands of road miles.

 

[madmac666]

Definately converted! Your thread about this subject (linked in my first post) was very helpful and gave me a direction to follow for the build so you have been instumental in my project from an early stage. Thank you for the tips.

To five you an idea of how quick the car is now - it does a standing 0-100mph in 4.6 seconds, not bad for a 2.2l 4-pot engine. The engine is now almost 5 seasons old and is showing signs of wear. It's probably only covered 5000 miles in that time, but they have been very hard miles. I intend to either freshen it up over the winter or build a new engine. Having the required funds will decide which route i go there.

One thing i wanted to ask - all through the year of development, the engine has produced quite a lot of black smoke on boost. We had this down to the damaged SC and balance being out from the original mapping we did but now the new SC is fitted and the map has been corrected, the fuelling is at 12.4:1 on boost so we know it's as lean as it can be and still be safe. One theory we're looking at is the smoke is being produced because unburnt fuel is going straight out the exhaust and burning in the manifold. This would then indicate the 272 cams i have may have too much overlap. We had originally thought we should try a pair of 282 cams but if i'm right in my assumptions of why the black smoke is so prominent, 282's would make the problem worse. Going to a set of 260 degree cams seems like a retrospective step for torque and power however.

Do you have any views on this?

Cheers

 

[Warpspeed]

Black smoke can be a result of granny driving, and then really nailing it.
The sudden heat gets a lot further down the exhaust pipe, and a lot of soot and crap can be blasted out.

This can create quite an impressive black haze in the rear view mirror if you normally drive around at legal speeds, and suddenly call upon a bit of real acceleration.

With boost pressure normally quite a bit higher than exhaust manifold pressure, (and a good four valve head), a little valve overlap goes a VERY long way.

The docility and tractability of twincharging (or supercharging) can easily mask excessive valve overlap.
It is not like a cranky highly tuned high compression n/a engine with bulk exhaust reversion at low rpm.

Another way to attack this problem might be to set the injector opening time to be after exhaust valve closing.

This works, until you start getting up to very high injector duty cycles, where the injectors might be spraying almost constantly flat out. Larger injectors will fix that.

Blowing a bit of cold air over the exhaust valves cannot be all bad, but blowing fuel out can be a financial hazard.

 

[madmac666]

The black smoke is evident all the time the engine is on boost. Since the car is used for competition only, it's never driven gently but i know what you mean about buildup of soot in the exhaust from granny driving. I'm not at all concerned about fuel economy as the fuel cost is minuscule compared to the costs of running the car and getting to/from events.

It's not a problem really, just unusual to see it now the mapping has been done.

I hope to post some out of car videos soon. My season is almost over and then we'll be into the winter rebuild in advance of the new season starting in April next year.

Mr Warpspeed, where are you based?