TERS strategy and Wastegates.

[gruntguru]

Let's start with some hypothetical figures at WOT, 10,500rpm.
Crankshaft power - 450kW
MGUH power - 70kW
This 70kW can be sent directly to the MGUK to make a total of 520kW. Of course the rules allow a max of 120kW to the MGUK so if the batteries are charged another 50kW could be drawn from the batteries to give a total of 570kW to the wheels. So this is the maximum available for propelling the car. Or is it?

What if we have a waste-gate that can be opened to reduce back pressure on the ICE while still producing sufficient boost by driving the compressor with the MGUH? What if the crankshaft power under these conditions is now 470 kW due to the reduced pumping loss in the exhaust stroke? We would then have 470kW plus 120kW from the MGUH (powered entirely from the batteries (ES)) for a total of 590kW.

So which teams run a waste-gate? It doesn't make any sense to include one, since boost is controlled by the MGUH, collecting energy which would be lost by opening a waste-gate. It has been suggested that the waste-gate is there as a backup boost control - an intake blow-off valve would be smaller, simpler, cheaper and lighter. Is the waste-gate being used for a short-term power increase? A clue would lie in the size of the waste-gate valve. To use it in the way I have suggested, it would probably be larger than you would expect for boost-limiting alone.

[henry]

Seems a good strategy. Essentially at max power you have an ICE with an electrically powered supercharger. At lower power demands the wastegate is closed and the exhaust turbine (ET) supplements the supercharger and at higher, but not max, the ET provides power to the energy store (ES) or MGU-K.

This of course requires a stratagem for replenishing the ES to make best use of this technique over a lap. Perhaps deploy max power at the beginning of a straight and recharge the ES towards the end. Slightly lower top speed but get there quicker.

[gruntguru]

Over the course of a race it may not even be desirable to use this strategy every lap. It would be a fuel-thirsty strategy for one thing. Consider it emergency max power - used for qualifying, overtaking, defending perhaps.

[henry]

Over the course of a race it may not even be desirable to use this strategy every lap. It would be a fuel-thirsty strategy for one thing. Consider it emergency max power - used for qualifying, overtaking, defending perhaps.

Agreed, nothing in modern F1 is used to the maximum every lap. But when it is used there will still be a need to recharge the ES. I suggested a simple, all purpose, strategy.

I'm not sure that it would be fuel-thirsty though. The fuel rate is fixed. My suggestion is to borrow fuel, and hence power, from the end of the straight and use it at the beginning. This is a sort of re-imagining of the method Red Bull have used in the past using lower gearing to have higher acceleration and lower top speed. In their case the limit at the end of the straight was the rev limiter, in this case it would be reduced power to the wheels because of the need to charge the ES.

I guess there would be a higher demands of the ES and it's circuitry which makes another of the huge number of trade offs in the design of these systems.

Oh and I don't know whether or how the system would know where it is on the straight.

[gruntguru]

Yes, i certainly wasn't disagreeing with your strategy.

I said "fuel thirsty" meaning a large drain on ES power in addition to the max fuel rate. The ES would be supplying 120kW to the MGUK plus sufficient power to the MGUH to run the supercharger. Any demands on the ES usually need to be paid back in lower fuel efficiency at some other point on the track. In reality it would probably be necessary to maintain some power production in the turbine at the penalty of some (lower than normal) back pressure.

Yes.

Such a system would offer a temporary power boost - perhaps via a "push to pass" button, clever programming or both.

[henry]

I think you have raised an interesting possibility for a further mode of operation of these hybrid power units.

It seems very likely that at the current state of the art that, as you say, it would be implemented as an overtake or some other low duty cycle function. But I would imagine that if it does give a performance advantage the race would be on to increase the duty cycle over time. I'm unclear on whether the electrical components are homologated in the same way the ICE is.

Do you have any feel for the power needed to drive the compressor at 10500 or the power fom the MGU-H in self sustain?

[gruntguru]

At 3 bar MAP and 16:1 AFR and 100% efficiency, the compressor power is 44kW. Divide by the efficiency to get actual compressor power e.g. for 80% isentropic efficiency the power is 44/0.8 = 55kW. Higher AFR gives higher airflow and proportionally higher compressor power. e.g. at 18:1 AFR and 80% efficiency the power is 55 x 18/16 = 61.9kW.

I will think about the MGUH power tomorrow.

[gruntguru]

Turbine power at 3 bar, 10,500 rpm is approximately 160 kW x turbine isentropic efficiency. eg 128 kW for an 80% efficient turbine. This does not include blowdown energy which at a guess could be another 50 kW.

Interesting. 178 kW harvested minus 62 to drive the compressor - pretty close to 120 kW - now where have I heard that number before?

On the subject of boost pressure. Provided the ICE can run lean enough without losing power, this "emergency power" mode would be operated at the highest boost possible - why? Because electric supercharging can produce additional crankshaft power by pushing the pistons down during the intake stroke - negative pumping loss if you like. So this might be another way to get additional power from the ES beyond the 120 kW limit set by the rules.

[henry]

Thanks GG. These numbers suggest that most of the time the PU can provide max power from the MGU-K without troubling the ES, which would be used to recycle braking energy to help fill in holes in torque and boost at part throttle conditions. It could also provide energy for occasional use of your supercharge mode. Or perhaps a bit more, 4MJ at 62KW is about a minute I think.

I had hoped to do a calculation of how much time a burst of supercharge mode would save at the beginning of a typical straight but my rusty maths failed me. I guess it would be more than zero and less than last years KERS. Still worth having.

[andylaurence]

At 3 bar MAP and 16:1 AFR and 100% efficiency, the compressor power is 44kW. Divide by the efficiency to get actual compressor power e.g. for 80% isentropic efficiency the power is 44/0.8 = 55kW. Higher AFR gives higher airflow and proportionally higher compressor power. e.g. at 18:1 AFR and 80% efficiency the power is 55 x 18/16 = 61.9kW.

I will think about the MGUH power tomorrow.

Interesting stuff! How do you calculate the power to drive the turbine?

[gruntguru]

You make a lot of assumptions Here is an example.
- Fuel flow is fixed at 100 kg/hr = .0278 kg/s.
- At 18:1 AFR the airflow is 18 x .0278 = 0.5 kg/s
- Exhaust flow = fuel flow + airflow = 0.5278
- Exhaust temperature (before turbine) = 850*C = 1123*K
- Exhaust back pressure = 3 bar abs
- Pressure after turbine = 1 bar abs
- Assume exhaust is an ideal gas (a bit dodgy)
- assume isentropic expansion ie T2/T1 = (P2/P1)^((k-1)/k) . . . . k=1.4 for air
solve for T2 and calculate power using Power = mass flow x Cp x (T1 - T2) . . . . . (Cp = 1.005 for air)

Or you could do what I did and find a calculator on the net and plug in the numbers. http://www.engineering-4e.com/calc4.htm The "Isentropic Compression" calculator works for compression or expansion since one is the reverse of the other.

[andylaurence]

Thanks for the explanation. I was intrigued from the figures people give about "it takes this much power to spin the supercharger" that always seem to be quite high. By that maths, it only takes ~14Kw to boost a 6.2L engine spinning at 6000rpm to 0.5bar of boost. Mine barely needs 5Kw! Given that a Rotrex is allegedly 97% efficient, that's not a lot of power required to spin it (~7bhp) for half a bar of boost.

[Tommy Cookers]

yes, this reminds us that ....
the 6L needs only 3L's worth of work by the supercharger for 0.5 bar boost (1 bar charging is from natural sources)
2014 F1 might only need 0.8 bar boost (it can get 1.2 bar charging naturally)
though a lot more boost is expected by many (targeting increased massflow or exhaust increased pressure or both)

low boosted SI engines take only about 2-3% of crankshaft power to drive the (centrifugal) supercharger
and gain mechanical efficiency (by upsizing the power with little increase in the mechanical losses)
with little loss in thermal efficiency as CR needed little reduction from the NA version's
and some of the power driving the the supercharger is 'crankshaft-recovered' due to the boost 'forward pressure'
the best efficiencies ever in aircraft engines (of conventional type) were so obtained around 1928-35
then they worked out that WW2 was not to be run on an efficiency formula

though ..... still with quite low boost ....
NACA tests in the early 1940s also showed that recovery turbines could take up to 30% of the combined power
by raising the turbine load even into the region that gave true back pressure
such raised exhaust pressure caused power to migrate from crankshaft to turbine and required suitable timing of EV closure
combined power was not improved but overall efficiency was, presumably due to better exhaust pressure conservation in blowdown

[gruntguru]

Thanks for the explanation. I was intrigued from the figures people give about "it takes this much power to spin the supercharger" that always seem to be quite high. By that maths, it only takes ~14Kw to boost a 6.2L engine spinning at 6000rpm to 0.5bar of boost. Mine barely needs 5Kw! Given that a Rotrex is allegedly 97% efficient, that's not a lot of power required to spin it (~7bhp) for half a bar of boost.

97% is probably the claimed mechanical efficiency of the step-up drive (are they still a friction drive "gearbox"?). The isentropic efficiency of the compressor itself is unlikely to be much more than 70% - and less under most conditions.

[dren]

Thanks for the explanation. I was intrigued from the figures people give about "it takes this much power to spin the supercharger" that always seem to be quite high. By that maths, it only takes ~14Kw to boost a 6.2L engine spinning at 6000rpm to 0.5bar of boost. Mine barely needs 5Kw! Given that a Rotrex is allegedly 97% efficient, that's not a lot of power required to spin it (~7bhp) for half a bar of boost.

97% is probably the claimed mechanical efficiency of the step-up drive (are they still a friction drive "gearbox"?). The isentropic efficiency of the compressor itself is unlikely to be much more than 70% - and less under most conditions.

Yes, they are still friction drive. It's a planetary gear set friction drive.