Supercharger / turbocharger shaft horsepower

M

maticulus

Member
The BorgWarner bot calculator lists shaft horsepower for a given boost pressure. I just had a thought regarding this while considering what motor I want to use in my finished assembly. Shouldn't those values be inaccurate for our application? For the traditional turbo the values were derived from, the compressor wheel and the turbine wheel are connected to the same shaft.

If as demonstrated the boost necessary to achieve the same performance using an electric turbo relative to a traditional turbo is about half as much, given the absence of the parasitic power draw associated with a traditional turbo through exhaust restriction is no longer present, isn't it reasonable to conclude that the shaft hp normally required to achieve a certain power level with a traditional turbo is also reduced for an electric turbo?

It has to be because the 500 traditional turbo hp has additional boost pressure necessary to account for the efficiency loss to exhaust restriction in route to the power goal. Unless in testing to arrive at the energy input level the compressor wheel alone is tested on a bench rather than in real-time on a motor and instead the actual power input measured from that setting.
 
The shaft power to make say 10psi of boost at say 70% compressor efficiency will be the same at the same mass flow irrespective of whether there a turbine is driving it or an electric motor. However the electric SC may be flowing more mass at the same engine rpm/boost due to lowered exhaust backpressure (if there is significant exhaust overlap). Probably not enough difference to throw the Matchbot calcs out by much IMO.
Of course, as you point out, the power made by the engine with that 10PSI will be significantly more and so less boost is required to make the same engine power and therefore less power is required from the electric motor.
 
Last edited:
brettus8 said:
The shaft power to make say 10psi of boost at say 70% compressor efficiency will be the same at the same mass flow irrespective of whether there a turbine is driving it or an electric motor. However the electric SC may be flowing more mass at the same engine rpm/boost due to lowered exhaust backpressure (if there is significant exhaust overlap). Probably not enough difference to throw the Matchbot calcs out by much IMO.
Of course, as you point out, the power made by the engine with that 10PSI will be significantly more and so less boost is required to make the same engine power and therefore less power is required from the electric motor.
Click to expand...
In other words as I've projected, efficiency goes up for the entire system relative to the traditional turbo, more power generated for less input into the system.
 
I am just starting to wrap my head around this electrical supercharger idea, and this post intrigues me. I have always thought that turbos were not free power as many have claimed due to the exhaust restriction. I watched a video (can't remember who put it out there) where they did a dyno run on a normally aspirated motor and got a baseline. Then they connected a supercharger to the crank as normal, but did not connect the supercharger to the intake. There did different size supercharger outlet restrictions to simulate actual boost pressures and performed dyno runs to determine the actual amount of HP it took to run the supercharger. I can see the benefits of electric supercharging to do away with turbo lag, and being able to easily change boost levels compared to turbos and superchargers. The videos I have seen so far are people doing drag racing runs, and they drive the supercharger with pre-charged batteries so they don't produce any load to the engine, just boost. My question is what has less engine parasitic losses: Turbo with exhaust restrictions, Supercharger with crank driven losses, or Electric supercharger with charging system losses (as I want a full time electric supercharger set up)?
 
climbing96 said:
My question is what has less engine parasitic losses: Turbo with exhaust restrictions, Supercharger with crank driven losses, or Electric supercharger with charging system losses (as I want a full time electric supercharger set up)?
Click to expand...
Do you need boost full time, and what is full time, is it a 20-minute track day session or a 3-hour drive through traffic?

If you need it available full-time but not on all the time then think about a second alternator just for charging the boost batteries. I haven't finalised my design setup yet, but I am planning to do LTO batteries and a second alternator. My track usage will be in a time-attack type setting, so I only need boost for a few minutes on a couple of hot laps.
 
I do not want to hi-jack this thread, but the info is interesting. My situation is that I am building a twin Hayabusa motorcycle engine powered off road buggy. Due to the nature of off roading, the amount of boost and time is never known. I plan on long days off roading through the mountains, across the dunes, or playing around at close by off road parks. Some high RPM adventures, and other times, low RPM rock crawling. I am doing away with the magneto and installing at least one alternator on each engine to run the engines, lights, winches, etc. I am curious to the parasitic draw, feasibility, and reliability of electric. I have seen some impressive videos, but they were being used in short bursts. Also I want the most efficient use of boost as these engines don't equate to a big block V8.
 
climbing96 said:
I do not want to hi-jack this thread, but the info is interesting. My situation is that I am building a twin Hayabusa motorcycle engine powered off road buggy. Due to the nature of off roading, the amount of boost and time is never known. I plan on long days off roading through the mountains, across the dunes, or playing around at close by off road parks. Some high RPM adventures, and other times, low RPM rock crawling. I am doing away with the magneto and installing at least one alternator on each engine to run the engines, lights, winches, etc. I am curious to the parasitic draw, feasibility, and reliability of electric. I have seen some impressive videos, but they were being used in short bursts. Also I want the most efficient use of boost as these engines don't equate to a big block V8.
Click to expand...
This is a highly diverse and customizable option. Superchargers draw more parasitic power than a turbo because they're always driven by the crank unless the system has a clutch activated pulley. Turbos are considered free hp in error bc they are driven by the exhaust gases after the chemical energy has been extracted. The problem of the exhaust back pressure that results from driving them is ignored but it's significant considering the typical turbocharged non race engine sees as much or more exhaust back pressure than boost pressure at peak levels.

That can vary based on turbine housing size, the larger the turbine housing all else the same the lower the parasitic load as the exhaust back pressure is not as high however in exchange for reduced back pressure there is an increased delay in spool up time due to the larger and less restrictive turbine housing.

An electric turbo system has to be matched to the application. A daily driver boost scenario must take the approach of a Tesla vehicle arrangement where battery capacity (amp-hr) is high, voltage is high, available motor rpm is high (and geared down for torque multiplication) and system current loads on the low side. For a motor bike your problem will be battery space.

You'll need to review and understand brushless motor specs; kv, current rating, number of poles, and battery supply ratings and tap into to RC hobby calculators to estimate based on plans what kind of performance range you can expect.
 
This is why I was so excited to join this forum. Excellent information!! You have given me more information to research into. This isn't a motorcycle I am wanting to do this to, but an off road buggy that has two motorcycle engines powering it. Although I won't have as much space as the trunk of a 70's Caddy, I will have more than a motorcycle;-) One of the reasons this post caught my attention is it is talking about shaft horsepower, and this might help me to decide whether to go with a twin screw supercharger or an electric supercharger. For the electrical supercharger, I wasn't planning on driving the system with batteries, but with alternators and affiliated electrical equipment. Another post I am watching is the one about water/methanol injection to control charge temperature. This forum is a wealth of knowledge!!
 
I don't know if alternators alone would cut it, and then you have the parasitic draw of the alternators, if your alternator is taking 25hp to drive to give you 350 amps to power your electric motors I think all that is doing is adding complexity and more things that can fail.

And you will probably need multiple alternators as finding a 350amp one might be impossible, so 3 or 4 of them at 100 amps 48v, also a dual electrical system, 6v or 12v for the buggy electrics and 48v for the supercharger.

Better off direct driving the supercharger from the crank in that situation I think. But that said I would love to be proven wrong 😆
 
It occurred to me that we are using the wrong tool, instead of a few alternators, steal from hybrid cars and their generators. Designed to work in an engine bay, so can take the heat and vibrations. They call them belt starter generators, as they can also be used in place of the starter motor, and can put out 400 or 500 amps

 
More info to research into. Thanks. I have been searching for a 48 volt alternator but my google foo wasn't getting it done. Now I know what to search for;-)
 
You must log in or register to reply here.
Top