WhiteBlue wrote:Tommy Cookers wrote:type056 wrote:
Thanks.But it is not seem MGU-H energy come from heat.
Correct
the turbine is driven by exhaust gas pressure, which is strongly related to velocity, but weakly related to temperature
unfortunately most of the pressure at the opening of the exhaust valve is dissipated, so is unavailable to the turbine
the turbine will be using the natural pressure pulses in the exhaust
it will recover in this way a useful amount of free power with little reduction in power from the pistons
most exhaust heat cannot ever be accessed by a tubine in the exhaust gas
that's why BMW have developed their TurboSteamer, using exhaust and coolant heat to make steam (ie at pressure)
to turn a steam turbine connected to the crankshaft
adding about 15% to the engine power for the same fuel consumption
Pressure and heat are connected in the working cycle of a heat engine. The temperature automatically changes with the pressure. If you compare the turbo engine with a NA engine you find that the available pressure behind the exhaust valves is not utilized. It is exhausted to the environment without generating work and that results in a hotter exhaust temperature. The turbo engine takes that available work and extracts it by building up a back pressure between the valves and the turbine.
there are many ways in which pressure and heat are connected in a heat engine
they are all less than ideal, some very far from ideal (particularly so in discontinuous processes ie piston engines)
blowdown takes place at 7 bar or more in such engines (and wastes much of the pressure in all such engines including turbos)
because the flow is supersonic
as many of its friends tell me, the turbo engine does not build up a (significant) back pressure between the valves and the turbine
any back pressure acts against the upward exhaust stroke of the piston (the exhaust valve is open all this time)
so must be avoided (ie by not overloading the exhaust with a heavily-loaded turbine)
Wright explain and show this as crucial, in the Turbo-Compound literature
this is of course why blowdown starts so early ie at 7 bar (earlier than that when there is a turbine in the exhaust)
in our sort of turbo or TC engine the turbine at best uses essentially the transient pressure pulse (which is good of course)
if we have a 'Hyperbar' engine turbocharged to about 15 bar induction pressure, the above does not apply
an ideal DI system (controlling combustion speed by injecting throughout combustion) might move the SI engine in that direction
BMW have not done their Turbosteamer work without good reason
they and others eg GM have made worthwhile progress with electrical generation from exhaust heat (Peltier effect)
a turbine would appear by your account (in part) to undermine these above (by reducing the exhaust energy available to them)