Variable Electronic Supercharger

[SSpeed]

I had a supercharged SBC where the supercharger RPM is fixed (60,000 rpm), it made about 7psi. With a better set of heads, bigger valves and camshaft, the boost would only reach 6psi. The SC still spun at 60,000 RPM at engine 5500rpm, but the intake restriction was reduced and the engine made more HP despite less boost. The Supercharger was still essentially producing the same volume (This is not 100% correct because it is affected by backpressure, but it close enough). There was less restriction and more air moving into the combustion chamber. There was less of back up of air in the intake manifold track.

 

Don't take this personal. I think many people do not realize this. There is a lot of misinformation on the internet and overly simplified explanations.

 

No offense taken, enjoying the conversation.

 

Did you have a roots or centrifugal supercharger? The reason your pressure went down was because your engine was consuming more air volume. All power adders can produce a certain pressure at a given volume. When volume increases beyond what they can give, pressure goes down. That's why you see smaller turbos choking at higher RPMs.

 

If you had a Procharger, this is what they say:

 

https://www.procharger.com/centrifugal-supercharger

 

The centrifugal supercharger contains an impeller which spins at high speed to draw air into a small compressor housing (volute). When air leaves the impeller, it is traveling at high speed while having a low pressure. This low-pressure, high-speed air is sent through a diffuser which converts the airflow so that it is high-pressure, and low-speed. The air is then fed into the engine, where the additional airflow (caused by increased pressure) gives the engine the ability to burn more fuel and have a higher level of combustion.

 

DIFFUSER

Located between the impeller and the volute is the diffuser. Downstream of the impeller in the flow path, it is the diffuser's responsibility to convert the kinetic energy (high velocity) of the gas into pressure by gradually slowing (diffusing) the gas velocity.

 

So I guess I had it partially wrong too. The impeller speeds up the air into high velocity/low pressure, while the housing itself converts it to low velocity/high pressure, but it's still happening inside the power adder housing. Without that, you aren't getting any appreciable boost.

 

The intercooler is in place to act as a heatsink to extract heat from the air, heat that is a result of the air being compressed. If air were compressed simply by backing up in the engine, the location of the intercooler would be useless.

Edited October 10, 2019 by SSpeed

[dgoodhue]

If this were true, a 3-port EBCS would not work. The way a 3-port EBCS works is that the boost signal from the compressor housing port goes to the EBCS port, which then modulates the signal to the wastegate allowing it to open at the desired level. If it were true that the air is not compressed in the compressor housing, then you would never see compressed air at the wastegate either because the only signal the wastegate is getting is coming directly from the compressor housing line.

 

Is what Western Turbo says wrong?

 

https://www.westernturbo.com/blog/how-does-a-turbocharger-work-anyway/

 

The western turbo website gives me a security error so I didn't go to that website.

 

"Compressors are the opposite of turbines. They consist of two sections; the impeller or compressor wheel and the compressor housing. The compressor wheel is connected to the turbine by a forged steel shaft. As the compressor wheel spins, air is drawn in and is compressed as the blades spin at a high velocity. The housing is designed to convert the high velocity, low pressure air stream, into a high pressure low velocity air stream, through a process called diffusion. In order to achieve this boost, the turbocharger uses the exhaust flow from the engine to spin a turbine, which in turn spins an air pump. The turbine in the turbocharger spins at speeds of up to 150,000 rotations per minute (rpm) that is about 30 times faster than most car engines can go."

 

Cummins seems to say the same thing as well.

 

https://www.cummins.com/components/turbo-technologies/turbochargers/how-a-turbocharger-works

 

The compressor also consists of two parts: the compressor wheel (5) and the compressor housing (6). The compressor’s mode of action is opposite that of the turbine. The compressor wheel is attached to the turbine by a forged steel shaft (7), and as the turbine turns the compressor wheel, the high-velocity spinning draws in air and compresses it. The compressor housing then converts the high-velocity, low-pressure air stream into a high-pressure, low-velocity air stream through a process called diffusion. The compressed air (8) is pushed into the engine, allowing the engine to burn more fuel to produce more power.

 

I agree with Cummings, but they are talking about a intake as system. Without any restrictions the turbo will not make compressed air at its outlet.

 

Maybe I wasn't clear on what I was writing. I will use an example. On stock Legacy GT, the stock boost is ~12psi @ the intake manifold. The intercooler and pipes (length of pipe and bends) cause resistance in the intake track. Using an assumption the boost drop across the intercooler, bends and length of piping is 3psi, which probably not that far off. The pressure before these at the turbo compressor outlet and in the turbo compressing housing past the blades is likely 15psi. I don't know if you have ever had a boost leak, but with a relative small leak (like the IC to TB coupler) the boost practically goes to zero. The boost level at the turbo compressor outlet would also go close zero (there maybe a small level of boost due to the restriction of the IC, Intercooler pipes)

 

Ignoring the engine air intake demands, the turbo charger compressor is pressuring the intake track by filling it with air, the air becomes compressed due the volume of air at the outlet of the turbo. A turbo charge is optimized to run in these conditions (as opposed the leaf blow supercharger) In the case of properly sized turbo, the turbo supplies enough air to maintain the compressed air & the air being used by the engine.

 

In the case of undersized the turbo, the engine will use more air than turbo can supply, so it will not be able to maintain the boost level, maybe even not maintain any boost pressure at all, and a worse case of not being able flow enough air to even supplied the engine Naturally Aspirated air flow demands. The last of what is likely happening in the Mustang GT video you posted.

 

The eSC in the video uses a compressor housing that looks like turbocharger. Assuming that is functional and not just for show, the eSC should make pressure on a close intake if spun fast enough. The electric motor is likely an issue. the compressor housing may be too small for anything but the smallest automobile engine. [The successful eSC I have seen are all running of more voltage than 12V battery.] An interesting test would be see if that eSC would make boost on lawnmower engine powered gocart.

 

On aside note a regular fan can make positive pressure (boost). I have seen in industry. (I used to worked as field engineer as for company that made sensors for industrial process filtering)

Edited October 11, 2019 by dgoodhue

[dgoodhue]

Did you have a roots or centrifugal supercharger? The reason your pressure went down was because your engine was consuming more air volume. All power adders can produce a certain pressure at a given volume. When volume increases beyond what they can give, pressure goes down. That's why you see smaller turbos choking at higher RPMs.

 

I had a Paxton centrifugal SC, but yes that is what happens.

[SSpeed]

We are saying the same thing I think. A turbocharger pressurizes the intake tract when it overcomes the vacuum of the engine, so in that sense I see what you are saying. The actual compression happens in the compressor housing though, in the tiny little diffuser section that's after the impeller and before the snail portion of the housing, right? If the engine uses more volume than the turbocharger is providing, or there is a boost leak, the compressed air dissipates rapidly. Same thing happens when you open a hose on an air compressor, the compressed air dissipates rapidly to atmosphere, but pressure it still there prior.

[cww516]

As I had it explained to me by a hydraulics engineer (and air is a fluid)- pumps don't make pressure, pumps make flow. Restrictions are what cause pressure to build up upstream, or if you want to think about it in the opposite direction, pressure drops across them (see also: flow across an orifice plate). Maintaining constant manifold pressure (positive or negative relative to atmosphere) is just a steady-state flow condition where the engine is consuming as much air mass as the turbo is providing. Turning up the boost is another way of saying that you're increasing mass flow rate without changing your restrictions, and some newer cars (Focus RS comes to mind) don't actually use manifold pressure as a target at all when calculating requested torque because it's easier to consistently make the same numbers (no manifold temp sensor needed).

 

Also, "compression" happens as the air gets slung radially outward along the vanes of the compressor wheel- it's getting pumped from the centerline of the wheel toward the outside, and the turbo outlet is just the path of least resistance for that relocated air. If you look at a section cut, you'll see that the outer passageway continually increases in area as you work your way around toward the outlet. The air compressor hose analogy isn't great in this instance, since the throttle plate is much more restrictive than the friction between the air flow and the walls of the charge pipe and/or the intercooler (dgoodhue's 3psi guess is actually a little high, there's a chart here), but it's definitely not inaccurate. The air compressor hose thing is the same concept as breathing through a straw- the smaller and longer the tube is, the more restrictive it is due to friction losses. Given a small/long enough hose (and big enough fitting bores upstream), you'll actually get to a point in the hose where you approach supply pressure, even with the end wide open.

 

 

Hopefully something in that rant made sense to someone, I saw an opportunity to nerd out and went for it.

[FLlegacy]

https://newatlas.com/garrett-turbo-promises-more-power-and-efficiency-less-lag/

 

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