Late to the game, but better late than never…
matt21 wrote:Tommy Cookers wrote:
The MGUH (or TERS) is turbocharger connected to an electrical motor/generator.
It works in response to pressure in the exhaust flow, not to heat as such, thus much of the 'TE' is unrecoverable. Much of the residual cylinder pressure is lost in the 'near-instantaneous' blowdown, and is unavailable to a turbine.
I would like to contradict here.
A turbine is, if well designed, not only using pressure of the exhaust flow to generate work, but also a certain amount of the temperature.
Furthermore, as BMW has done in 1983 with the M12 and with the actual twin-scroll N55, the turbine is using the kinetic energy by leading seperate exhaust lines into the housing.
A turbo is primarily a waste heat recovery system. This heat energy is 100% wasted in a naturally aspirated engine in so much as it is not captured and utilised.
The turbine wheel marginally (15% to 20% maximum) is driven by the kinetic energy in the exhaust as a result of backpressure behind the turbine (pressure differential across the turbine) blowing through the housing. However, it is primarily driven by the thermal energy imparted to the turbine blades as the hot exhaust gas (1025°C+) expand through the turbine nozzle area and cool. As the gas enters the turbine housing it is allowed to expand in the volute surrounding the turbine blades. As the hot gas expands, it cools and a significant proportion of the thermal energy (anything up to a 200 degree temperature loss can be experienced across the turbine) contained is released through this expansion and thermal release. This is why exhaust temperatures post turbo are typically much lower than those seen on a naturally aspirated engine.
Turbo engineers I have spoken to quote that 80% to 85% of the energy used to drive the turbine (in a single scroll turbine housing) is derived from the thermal energy in the gas rather than the pressure differential across the turbine. The parasitic loss due to back pressure that is incurred by driving the turbine is typically in the range of 0.1% of engine power per psi exhaust back pressure. So 20psi back pressure due to the turbo would result in a 2.0% reduction in engine power as a result of pumping losses transmitted to the crankshaft. When compared to the 15% to 20% parasitic loss of a direct supercharger, it is evident of the advantages of the turbo from an efficiency aspect.
What twin-scroll (TS) turbine housings attempt to overcome is the inherent shortcomings of the single-scroll design, primarily by separating those cylinders whose exhaust gas pulses would otherwise interfere with each other. This provides benefits similar to those of pairing cylinders on tuned length exhaust manifolds to prevent pulse interference and gain an improvement from pulse wave reversion.
TS housings specifically provide an advantage by pairing “like” cylinders to separate sides of the turbine housing such that more of the the kinetic energy (up to 25% or 30% of the total energy derived) from the exhaust gases can be recovered more efficiently by the turbine due to reductions in pulse interference. This means a 5% to 10% improvement in energy recovery from the exhaust gas and allows either earlier boost threshold or faster spool times in the same turbine A/R or the use of a larger A/R housing for increased top end power production without the typically associated increase in lag and poor transient boost response times.
Other advantages are superior scavenging effect from the twin-scroll design which provides more optimal pressure distribution at the exhaust port and more efficient delivery of exhaust gas energy to the turbocharger’s turbine. The effect of which is to allow greater valve overlap, resulting in an improvement in both quality and quantity of the intake charge. Coupled with overall greater VE and improved scavenging, more optimal ignition timing is possible reducing peak in-cylinder temps allowing leaner AFR’s as less fuel is required for evaporative cooling. TS turbos have provided between 7% and 9% increase turbine efficiency coupled with fuel efficiency improvements close to 5%.
Tommy Cookers wrote:I always had in mind that in heat engines there is many relationships between heat and pressure, when heat manifests itself as pressure it can do work via a turbine (or piston). The FIA is creating public confusion with its chosen terminology TES and MGUH.
Agree, heat energy contained in high pressure gases can be harvested through rapid expansion of the gas in a controlled environment. This allows the thermal energy to be transferred through some mechanism (enclosed turbine wheel) through the expansion process.
Tommy Cookers wrote:Certainly turbos are (mostly) designed around trying to catch whatever they can, as you show with the BMW.
The big (philosophical) question is how much (of what) do they catch (when they say KE this may be an intentionally careful claim).
I should love to see a paper that clarifies this, ie with real time measurements from port to tailpipe.
As above, 70% to 85% of the energy used to drive the turbine is derived from thermal energy in the exhaust gas (depending on the type of turbine design) with the rest coming from kinetic energy resulting from the pressure differential across the turbine due to back pressure. The increase BMW quote in likely the increase in kinetic energy recovery through the use of a twin scroll turbine housing, which could be in the order of 5% to 10% of total energy used to drive the turbine. A substantial improvement.
Tommy Cookers wrote:The non-turbo engine is making these losses anyway, and the turbo doesn't have to be driven by this lossy part of the exhaust energy to work. There are turbos/turbines that are designed to work only on the steady (residual) pressure.
In large part the turbo works by boosting the massflow without increasing frictional losses. Even mechanically driven superchargers can improve efficiency in this way, it's not a mystery.
I’m not sure I understand what you are saying here. There is marginal kinetic energy transferred to the turbine to do work (as a % of power used). Their operation is not based on pressure differential. Again as above, parasitic loss due to backpressure transferred to the crankshaft as pumping losses are typically 0.1% of engine power per psi of exhaust backpressure. Direct drive superchargers incur a 15% to 20% parasitic loss due to their operation. Turbo efficiency comes from the limitation of parasitic pumping losses, no increase in direct drive parasitic loss and recovery of wasted thermal energy from the exhaust gases.
Tommy Cookers wrote:The BMW was the first turbo winner of the WDC (1983?), I think they had some naughty fuel, Renault felt robbed, and a discreet fuel war started. I wonder if this will happen again, the same 2 countries ?
(the fuel regs seem to have been opened up)
With the fuel regs opening up it certainly could be interesting!!
pgfpro wrote:IMPO I think that in a fuel-limited but not boost limited F1 that WI would be not very useful. If it was boost limited I could see the engineer's running a smaller compressor on the turbo and running a pre turbo WI. This will increase the efficiency of the turbo and make the compressor act like a slightly larger unit. But with a unlimited boost rule I see the engineer's running turbo's that have high efficiencies all the way to the choke line.
Not sure I 100% agree. As the maximum fuel flow is a known quantity, a theoretical maximum of air for combustion is known. All other variables will centre on this cap on engine power. What I surmise will happen is that compression ratios will be increased to the point where the fuel used (maximum octane rating) and maximum air flow requirements (compressor sizing) that the fuel flow will support do not result in detonation. The variables such as charge cooling, turbine A/R sizing, lag, boost threshold, compressor spool time etc will be the variables that individual engine suppliers with work through. If WI would allow higher CR’s to be used before onset of detonation then is would be a benefit as the max fuel flow could then in theory be used only for power on not cooling. There are also the packaging advantages in removing or limiting the compromise in space requirements for large, high aero drag and potentially heavy intercoolers and the associated long turbo to plenum pipe work. As the max fuel flow is know, then WI would be advantageous to take full advantage of the fuel for energy production.
pgfpro wrote:I have ran both post and pre-turbo WI and it was more of just a band-aid for not running a high enough octane fuel and also I ran it when I was running a inadequate inter-cooler system. But it had some disadvantages. One major when road racing I would often end up with puddling and notice surging. This was a 50/50 mix H2O/Meth.
I did use it at one point before adding larger injectors to help with adding more fuel but I experience uneven fuel distribution.
In a road cars and amateur motorsports, WI (and 50/50H2O/Methanol) is typically used as a band-aid solution for poor octane quality fuels, inadequate intercooling or excessive CR’s. For a professional race engineer where there are specific constraints due to regulations but no monetary concerns (such as in WRC previously and now F1 from 2014) WI could be used as part of a total solution so as other variables could be maximised, such as CR, or better powerplant packaging could be utilised. Either way, it still carries a number of downsides which you’d need to weigh up before relying on it as an integral part of the engine.
Never approach a Bull from the front, a Horse from the back, or an Idiot from any direction
