How to make power with a restricted engine?

[albino_insect]

I was thinking the other day, with so many series that require intake restrictors there must be some good technical information about the peculiarities of making practical power with a restricted engine. Does anyone know of any good technical papers on this subject? I would really like to find out how a restrictor plate effects a two-stroke engine. Thanks for the help.

[albino_insect]

I was thinking the other day, with so many series that require intake restrictors there must be some good technical information about the peculiarities of making practical power with a restricted engine. Does anyone know of any good technical papers on this subject? I would really like to find out how a restrictor plate effects a two-stroke engine. Thanks for the help.

[Jersey Tom]

What I'm curious about is with your restrictor, when you build your venturi you get a converging/diverging nozzle.. or something similar. If you manage to get a pressure drop accross it good enough to choke the flow at the throat, how do you keep your flow critically expanded as it leaves the nozzle travelling above Mach 1? What happens if you have overexpanded flow going into your intake manifold an engine, or likewise if you start propegating Prandtl-Meyer expansion and compression waves through there and into your engine. Or if that's even an issue since the whole thing is sealed.

Gonna have a talk with a couple of the top dog Fluids and Aerodynamics professors here bout that one.

[UQ Turbo]

Its bloody hard to have a supersonic exit in your restrictor. See Frank White's Fluid Mechanics textbook (Fig 9.12 P630). The exit pressure required for curves G,H and I are incredibly low. Now have a look at a turbo compressor map (as it's only possible to choke the restrictor with a turbo) and then do some calculations on pressure ratio required to sustain the engine at high revs and still make useful power. I think you'll find its not much of a concern: you'll get ample boost for power purposes up to 8000 - 9000 RPM - you can change gear after that...

I also did some 2D CFD on a choked restrictor which confirmed the theory in White.

UQTurbo

[DaveC]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">you'll get ample boost for power purposes up to 8000 - 9000 RPM - you can change gear after that... </div></BLOCKQUOTE>

Why not open a wastegate at that point and shift a couple thousand rpms higher? I havent looked into Cornell's ewastegate controller much, but I'd guess that was what they were going for. Without something like that, it seems you are just moving the powerband down much lower (and producing a lot of torque). IMHO, a turbos advantage is a wider powerband (seems like a very small advantage with a skilled driver), which is minimized if you must shift before 9000 rpm.

Whats up with your head mods UQ? Bring up that old thread and post some pics of your sleeves. I just havent found a solution to port shrinking I'm totally happy with, it seems epoxy is the only way to shrink the ports far enough into the head to be optimal, and we dont have a motor thats OK to do that sort of testing on right now. Maybe next year....

[Kwheels]

You guys want the real scoop on 600cc Ristricted engines. Call Jim Clifford @ clifford Cycles. He is the man. This guy builds these motors for a living and works with many 600cc Restricted operations. He has some tricks up his sleave that will get you extra HP? Ask about sponsorship ops. You want a little extra he has it. Here is his link. http://www.cliffordcycles.com

[Marshall Grice]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">as it's only possible to choke the restrictor with a turbo </div></BLOCKQUOTE>

How do you figure this? I believe pretty much every team is choking their restrictor at some point in the rpm band.

[UQ Turbo]

Well, our NA F3 couldn't pull enough vacuum to achieve the critical pressure ratio across the restrictor - I assumed that similar engines would be the same deal. Speak up if your NA engine chokes the restrictor! I'd be interested in how you could tell too...

UQTurbo

[Marshall Grice]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I'd be interested in how you could tell too... </div></BLOCKQUOTE>

*qualification* i'm not an engine guy.
I also don't have any MAP data in front of me.

My assumption is that pretty much any 600cc engine that can reach 12500+rpm can choke the restrictor based off of mass flow rate. I support that claim in our case by our dyno plot which shows a sudden torque decrease at ~12500. I'm sure others could come up with similar data or even show the eqn's if they were so inclined.

admittedly the name of the game is to choke the restrictor for a longer period of time(rpm wise) to maximise power output from the engine. So merly choking the restrictor doesn't in itself mean you're making the most power possible.

[Ben Inkster]

I think an NA engine will partially choke the restrictor in a single engine cycle but on cycle average it will not make full use of the maximum mass flow (unless very high rpms are used). I think anyone doing transisent CFD will come up with this conclusion.

I think the most popular restricted engine approach in racing is to run at very high rpm's with a large plenum to damp out transient air motion and get a steady choking of the restrictor and then use a high compression ratio to compensate for the lower volumetric efficiency of a choked air supply. I'm not saying I think this is a great solution for fsae, but it will allow a NA engine to induct as much air as possible.

-Ben

[Marc Jaxa-Rozen]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I think the most popular restricted engine approach in racing is to run at very high rpm's with a large plenum to damp out transient air motion and get a steady choking of the restrictor and then use a high compression ratio to compensate for the lower volumetric efficiency of a choked air supply. I'm not saying I think this is a great solution for fsae, but it will allow a NA engine to induct as much air as possible. </div></BLOCKQUOTE>

That's pretty much the approach we had with the 2004 and 2005 intakes, both had fairly large volumes (&gt;2.5L) and gave good power up to 14K, but at the expense of some throttle response (having no TPS compensation for most of that time didn't help, though).

Marc Jaxa-Rozen
École Polytechnique de Montréal

[albino_insect]

Making power is not just a matter of getting air into the engine. As rpm increases so do frictional losses which are a huge deal. Don't believe me? Renault F1 has been rumored to be as much as 2000rpm down on its competition at some times but still be able to make similar power and superior torque because of frictional losses. Also, I don't think I would agree with the large plenum approach. It takes alot of energy to accelerate air to the speed of sound or near the speed of sound so you may as well use the energy to get the air to your intake runners. In reality, I don't think any of us have done enough testing to really say any of this for sure. That is why I want to see a book or tech paper.

[BeaverGuy]

Making power may not be purely a matter of getting air into the engine but that is the dominant force.The frictional and pumping losses associated with increasing RPMs when the airflow doesn't increase will have an effect. If you have dyno curves with airflow plotted on them you can see where the losses start to dominate and you don't make more power, it isn't immediately after airflow flatlines but it is close.

The energy imparted to the air by getting it to sonic or supersonic velocities will still be there when you slow it down. It simply changes forms to something more usefull. High velocities aren't all that great because you have low pressures. The amount of air that you can induce is dictated by the pressure of the air in the plenum not the velocity of that air. Look at the gas dynamics and ideal gas equations and this is fairly evident. That is the reason that larger plenums will allow for greater power and airflow rates, the plenum better aproximates a steady state situation on the downside of the restrictor. An article by Gordon P. Blair about this and other issues with respect to restrcited engines can be found in the second Issue of Race Engine Technology. However, when they are examining large plenums it is 10 to 15 times the displacement of the engine and with the throttle(S) downstream of the plenum and restrictor not the 4 to 5 times that you might typically see on an FSAE car with a throttle upstream of the plenum and restrictor. I don't see the high RPM large plenum approach being beneficial to our situation because of the low speeds and throttle occuring before the plenum. Having to use increasingly larger final drive ratios to keep max speed suitable creates low speed traction problems while the having the plenum as part of the throttled volume causes throttle response issues. If the restrictor cut in at 8 or 9k RPM and the throttle was after the plenum then these approaches would be much more beneficial for FSAE.

If as I think you question is more about the topic in general then these would be appropriate solutions to the problem of a restricted motor. If you can find a copy of one of Gordon P. Blair's books on either 2 or 4 stroke engines and then also look at his article in RET this should give you a good idea of what you need to do.

[Garlic]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by albino_insect:
Making power is not just a matter of getting air into the engine. As rpm increases so do frictional losses which are a huge deal. Don't believe me? Renault F1 has been rumored to be as much as 2000rpm down on its competition at some times but still be able to make similar power and superior torque because of frictional losses. </div></BLOCKQUOTE>

Theories are not substantiated by rumours!! You can't prove any of what you said.

Airflow is still by far the dominant force in making horsepower.

As for making power with a restrictor, it's pretty simple. Work on maximizing air through the restrictor, otherwise all oher engine design still applies!

[Ben Inkster]

albino_insect, the purpose of the restrictor is to restrict air flow, and often the unique aspect of restricted engines are based on maximising air flow. This isn't to say that internal friction is not important, but it is a common problem with all race engines. I actually have a formula 1 book that states the pumping losses from the cooling system got to the point where the teams where attempting to detune the engines to reduce the cooling load and pumping losses and ending up with a greater effective power. It is an interesting point but one that fsae cars are not affected by (yet?).

BeaverGuy explains the gas dynamics well and I hope you understand his explanation! restricted engines still obey conservation of energy! The purpose of the plenum is to change cyclic air flow from the combustion chamber into steady state airflow through the restrictor because you don't want cyclic airlfow through your restrictor. A large plenum will partially reflect pressure pulses from the runners and inlet valve and hopefully prevent them from reaching the restrictor where they will mess up your perfect air flow pulling mach 1 and maximum airflow. Theoretically, only an infinite plenum volume will ahieve this but the larger the plenum the less significant the pressure pulses become. But as BeaverGuy has already said, having the throttle upstream of the restrictor effects your throttle response with large plenums.

Personally, I think the discussion of maximum power is purely academic and inappropriate for fsae where wide open throttle exists for maybe 5-10% of an endurance race. I think you are much better starting off with drivable torque curve and then start refining your max power.

[Jersey Tom]

It would make sense to me that with Mach 1 or better flow through your intake, head loss due to friction would start to have a noticable effect.

Which gives me an idea..

[albino_insect]

Great responses everyone. Just so everyone is clear on this, I'm not saying getting airflow isn't important for power. I know that it is extremely important, it's just not the only factor. I'll look into the books suggested. I mainly wanted to see if there were any outstandingly good papers or books done by maybe a sportscar racing team or a JGTC team.

[UQ Turbo]

Don't forget the effects of bore/stroke ratio, valve timing, runner x-sectional areas and lengths, port diameters, exhaust properties. These all play a massive role in where peak power and torque will come on in the rev range!

UQTurbo