We’re using the same criteria but are having trouble with the many variables in a very complicated process. Think carefully about my example. Work is done on the power stroke. If there is high residual pressure in the cylinder, as there is when the exhaust valve opens in a turbocharged engine, the heat energy is dumped into the exhaust and wasted. On the other hand, a NA engine with its larger relative expansion volume utilizes more of the heat energy in work and has lower pressure when the exhaust valve opens, all other things being equal.WhiteBlue wrote:You should start thinking in energy balance to understand the advantages of specific configurations. The higher the thermal efficiency of an engine the more power it makes from the fuel. In that regard it primarily matters how much thermal and kinetic energy is dumped with the via the exhaust gas and the radiators. One aspect already covered by TC is the frictional aspect. Turbocharged engines run on lower revs and waste less power in friction than higher revving naturally aspired engines. You need to look at the basic physical aspects to determine where your efficiencies are generated. That means comparing the main loss drivers. One other aspect of turbo charged engines is the better suitability to direct injection spray guided combustion. This type of combustion delivers the best fuel/air ratios and combustion efficiencies. But it works best at lower revs because the known injection systems are too slow for rpms beyond 9,000 rpm. There is work under way to improve the speed of injection but it is a long way to reach the 15,000 rpm the regulations allow for the new engines. So there are several good reasons to focus on the lower hanging fruits of charged engines.olefud wrote:Maybe under some unstated parameters. But what about the example of the two engines sized for the same power rating, one power cycle with the conventional engine at 100% volumetric efficiency and the turbo at 150% VE? The larger displacement conventional engine will have greater volume for power stroke expansion thus utilizing more of the heat energy. The turbo will finish work expansion with higher residual cylinder pressure and waste a bit more with an earlier exhaust valve opening for blowdown to relieve the higher pressure prior to the exhaust stroke.
The energy reclaimed by the turbo is not useful energy. Turbocharging has advantages but thermo efficiency wouldn’t appear to be one of them. The whole purpose of compounding is to reclaim this wasted heat energy.
The turbocharged engine, having higher combustion pressure as it approaches BDC must also open the exhaust valve earlier to allow for blow down from the higher pressure. This blow down energy is not trivial. The Wright R-3350 Duplex Cyclone operated its compounding turbine solely on blow down energy to avoid back pressure on the exhaust stroke. Just the blow down normally wasted energy harvested increased efficiency on the order of 18%. Aircraft engines are a special case but do illustrate a point. As an indication of relative waste energy, I’ve never known of a compounded NA engine.
Again, set aside the preconceived, conventional knowledge and rethink this. It’s a bit different but not that complicated.