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Supercharger horsepower

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4.4K views 27 replies 12 participants last post by  TNEngineer  
#1 · (Edited)
No, I'm not asking how much additional HP is added by a supercharger.

The laws of thermodynamics tell us that nothing is free. The belt is spinning the supercharger whether the wastegate is "using" the air or not. I haven't studied the airflow paths but I presume when the SC's output isn't being actively used, its output side is dumped to atmosphere to minimize backpressure on the SC, belt, and engine. BUT... there's still friction.

What is the effective HP consumption for a typical SC for engines in this size range? It cannot be zero. There is a cost for having the SC available. Does anyone know the power cost to have it standing by?
 
#3 ·
Wow, that seems extreme to spin freewheeling components with no backpressure. I can easily imagine that figure when it's compressing air against the engine intake. Then it becomes an analysis to confirm it's a net positive. But 20% just to freewheel....
 
#5 · (Edited)
The draw on a Hellcat was 180hp.
At peak only, not at low load.

10-20% is pretty typical I think, but that's a fraction of the total output. Sitting there in idle, making 1<hp, the supercharger lobes are spinning in vacuum. The friction of the moving parts gets converted to heat and it's pretty trivial.

EDIT: note that the pumping loss of the engine probably exceeds this.
 
#9 ·
I wonder why.
Yes, there is a bit more load on the alternator. Not 2%, and certainly not 20%.
More efficient is turbo, but you dont get SC boost immediately.
Electric turbo is where its at.

Soon there should be more information and testing, still new stuff. But if MB is using it, and its going in the Emira, my Elise will have them.
 
#10 ·
If you're asking why the power consumption is the same, belt vs. electric, it's because the same amount of work is being done regardless of the type of power source. If you want to express that in HP, you can also express it in electrical watts: Each HP is ~746 watts. Ultimately all power in the car comes from the engine, because even if you power a load electrically the engine generates that electricity via the alternator (either directly, or to recharge the battery).

There can be some minor efficiency differences from different power sources, but those pale in comparison to the amount of power being consumed. My guess is that belt driven accessories are a bit more efficient because they skip all the extra conversion steps from mechanical-to-electrical (alternator) and back through electrical-to-mechanical (electric motor driving the SC). No conversion is 100% efficient; the engine natively generates rotating mechanical power which is also what the SC needs. Adding two extra conversion steps in the middle adds inefficiency, period.

The key here is that all power originates in the fuel, which is converted to either mechanical (HP) or electrical (watts) power by the engine.
 
#11 · (Edited)
I thought the Mercedes electric turbo is more accurately described as an electric assisted turbo in that it still primarily relies on exhaust gas to drive the turbo. It was my understanding it uses the electric motor to help it spool up much faster to fight lag, giving much better throttle response, and building torque earlier. This seems like a great idea since it does not need to draw electrical power for the bulk of it's output and can still rely on otherwise lost heat/exhaust gas pressure. One never gets something for nothing though. What is being given up to drive a turbo? It seems too good to be true that it just harnesses otherwise lost potential.

The electric assist turbo seems like a great idea. Sorry to thread jack away from the supercharger question, which I'm also interested in: especially how much power the engine needs to generate and withstand in order to get it's output at the crank available to drive the wheels.
 
#12 ·
There's a phrase in construction that applies here. "It is what it is" The end justifies the means is another that comes to mind.
 
owns 2006 Lotus Elise
#14 ·
The answer to how much power is lost at low load can be "negligible":


AC compressors have been using clutches for decades (since 1954 as per wikipedia). The only reason not to use it on a SC is because the added inertia and complexity are not worth the gain in mpg. Since such clutches are well-understood at this point, the mpg gain must be very small. I don't have my SAE membership any more, but I bet they have a paper breaking it all down somewhere.
 
#15 ·
Our superchargers have a little bypass throttle, which means the air doesn't have to flow through them when they are not needed. I believe this makes them more efficient than the old 671 blowers which I don't think have that.
All blowers are doing a thing that is not directly related to the power they use, so they are not an over unity device
Turbos are using waste heat so they are most likely 'more efficient' than superchargers.
Internal combustion engines are wildly inefficient at turning fuel into rotational motion, usually less than 30 percent in the real world

Electric blowers have to take crank energy, turn it into electricity, then back to mechanical energy. While time wise it is useful, efficient it is not.
 
#18 ·
Turbos are using waste heat so they are most likely 'more efficient' than superchargers.
I believe turbos are driven by pressure. The heat is there but doesn't contribute to the power spinning the impeller, and in fact is a PITA to design around. It's certainly possible to extract work from heated air but I'm not aware of a TC design that does so. I'd love to be corrected if I'm wrong.

Spinning a TC in the exhaust stream adds backpressure to the engine, as noted by someone else in this thread. So there's no free lunch there either. I have no reason to believe that power extracted in the form of exhaust backpressure is somehow better than power extracted from the crankshaft.
 
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#16 ·
The supercharger is behind the throttle butterfly. When the throttle is low, and the pressure differential across it is low, it does little work.

I don't know what the numbers are, however, I did not notice much of a fuel efficiency difference in my car when I added the LotusSport supercharger. It dropped about 1mpg on average, however, it became much more fun, and the gas mileage at the track went down to about 6mpg from 8mpg when driving really hard.
 
#17 ·
I need some clarification. The Evora has a very visible wastegate on its SC. I presume its function is to dump unneeded pressurized air on the output of the SC before it reaches the throttle body. Doing so minimizes the load presented to the engine when the SC isn't needed, since RPM is not an indication of power demand.

Do the SC 4-cylinders also have such a wastegate? If not, how do they manage the excessive pressure when RPM's are higher but power demand is not?
 
#19 ·
The notion of turbos using waste energy isn't accurate, and as @IDEngineer said, they are driven by exhaust pressure, and they increase back pressure on the engine. We try really hard in N/A engines to minimize back pressure because it consumes power, and it's no different with a turbo. The gains from increasing intake flow offset the losses from increased back pressure, and also the higher pressure can increase the thermodynamic efficiency (search for Carnot efficiency).

Superchargers do the same thing, but you're always using engine power to keep a bunch of metal rotating. Turbos have much smaller rotational mass, and under low load, they find an equilibrium where they take just enough power from exhaust to keep the air flowing, so at low throttle, they can be more efficient than a supercharger.

Given the same power output, usually an N/A engine will be most efficient, because the turbo or supercharger are doing extra work compressing air, which the N/A engine isn't. It's complex, tho, because thermodynamic efficiency gains can offset this.
 
#20 ·
Can't the power draw of the SC be minimized by shunting its output to atmosphere with the wastegate? If the SC sees no backpressure (from the engine intake), all it does is generate waste heat in its bearings. Yes, it could be argued that it's "compressing" air, but if there's no resistance on the output side then there's no real compression happening and the load consists solely of the rotating mechanical mass.
 
#21 ·
It is still pumping air. Even with zero resitance, you have mass and acceleration (of the air) which equates to work done.
And yes, turbos use thermal energy and pressure, but the work done is more driven by the temperature drop. That’s the goal at least and is far more efficient than trying to drive with pressure.
 
#22 · (Edited)
I'm in a 757 flying from east coast to west, so I have hours of time to study this topic and all of the Lotus maintenance docs on my phone for casual reading. {grin} Those docs do not go into much detail of the SC's on the two engines and provide no illustrations of their interiors.

Fortunately, the Harry's Garage tour of his Emira build briefly mentions the V6's SC with a cover-off view of the internals. Here's a screenshot:

Image


It's pretty clear that the cover creates the air plenum that routes compressed air from the screws through the intercooler. That means the triangular opening showing the screws MUST be the output. This is contrary to what is stated in the video - he asserts that the airflow to the engine is out of the right side of this image, where the butterfly valve is located. But that cannot be true because 1) that would put the intercooler before the screws, and 2) the airbox+filter flow into the SC at that butterfly valve. Therefore I conclude this is a misstatement on his part. No biggie, but important to the operation of the SC system.

With ambient air coming in on the right, and pressurized air exiting the screws on the left, the function of the butterfly valve is made clear. It selectively creates a short-circuit around the screws. When closed, 100% of the air must pass through the SC, 100% of the air is compressed, and the SC sees maximum backpressure looking downstream toward the engine. But as you open the valve, the SC's inlet and outlet start to be directly connected. When a pump (of any medium) sees a short-circuit like that, its effectivity AND its workload are reduced. Basically it becomes less efficient which in turn consumes less power, the former normally a bad thing but in this case allowing the effective output of the SC to be varied in an analog fashion.

This differs from my earlier theory that the SC's output was simply dumped to atmosphere. But it accomplishes the same end result.

I presume the SC on the 4-cylinder works the same way, since the Lotus docs mention a relief valve. Sadly I don't have an internal view so I can't confirm.

This also means that the term "wastegate" is misapplied to the Lotus SC's. Those valves are not wastegates in the traditional sense.

My understanding continues to improve.
 
#25 ·
That butterfly is a bypass valve, vs wastegate on a turbo and functions via vacuum as you say. The TVS SCs do not have internal compression like a Whipple. In the TVS the screw thread pitch is constant in a whipple it the pitch gets tighter as you get closer to the outlet. The advantage to this on the TVS is lower HP loss and heating when in bypass. If I was to guess, the load of spinning the screws in bypass is very low, just overcoming the friction of the bearings, SC drive etc. In practice, I've not noticed a change in highway fuel economy of note on our SC elise. The addition of a taller 6th gear improved highway economy more than the SC hurt it.
 
#26 ·
If I was to guess, the load of spinning the screws in bypass is very low, just overcoming the friction of the bearings, SC drive etc.
That's my conclusion as well. My original presumption was that the valve vented unneeded pressurized air to atmosphere; since there's no "compression" without a closed output volume the workload (and hence power consumed) would be minimal. After seeing the opened V6 SC it's clear that the valve instead shorts the air pump's output back to its input, which has the same effect while keeping the air system closed (e.g. no openings to allow particulates to get into the system past the air filter).

Like you, I now suspect that the only real load presented by an "idle" SC (at any RPM) is frictional: Bearings, belt, friction of the screws meshing, that sort of thing. There is a small amount of "work" moving the uncompressed air through the screws but air has very low mass and when the pump is "short-circuited" there's no backpressure and thus no significant resistance caused by that air.