Quote:
Originally Posted by ChazInMT
Annnnd you totally dissed BMW's Valvetronic and some others Variable Valve Lift (VVL) systems which change the displacement of the engine by varying the intake valve to close before BDC on the suck stroke. In fact they keep the throttle plate wide open on those engines using the valves to "Throttle" the engine. It'd be somewhat difficult to retrofit your Geo Metro engines with it. Just sayin. Although I bet that magnificent b@st@rd MPGeo could pull it off....that guys a frickin genius.
|
Actually I think these are good case studies for understanding why a lot of manufacturers have not gone down this road. Every BMW today is equipped with Valvetronic, and thus has miniscule pumping losses, yet the fuel efficiency is not really stellar. My parents leased a new BMW hybrid X5 and its gas mileage really isn't much better than an MDX or RX350. It's better, but not by much.
Are BMW sedans much more efficient than Mercedes sedans? Nope.
Same with the Fiat Multiair, is the Fiat 500 getting far better gas mileage than a Smart or Scion iA? Don't think so.
Do the Nissan VQ37 cars get better gas mileage than Toyota 2GR or Honda J35 cars? Not really.
Pumping losses are only part of the equation. Mechanical friction is still the biggest culprit, which is why transmissions are getting more and more gears since that is the cheapest way to improve efficiency. It's worth remembering that pumping losses can only be a bit over 100kPa in specific torque, but friction can be several times that, and as you get into very low specific output ranges the combustion temperature is so low that the gas cycle efficiency itself is total crap.
For most cars, regearing them taller (with tires is cheapest) and running them a little leaner gets you most of the way for cheap. Tuning the engine lean can raise your manifold pressure by 10-15%, which translates into 1psi reduction in vacuum on the freeway for most cars. A reasonable cam will reduce the vacuum by maybe another 1psi. In some cases like my FR-S, there's some money left on the table with the VVT, and you can get another 0.5-1psi or so, and that's already half your throttling losses.
This is why Toyota has mostly ditched their variable lift approach and gone with very wide angle cam phasers in their newest engines.