View Single Post
Old 12-06-2010, 08:27 AM   #148 (permalink)
t vago
MPGuino Supporter
t vago's Avatar
Join Date: Oct 2010
Location: Hungary
Posts: 1,769

iNXS - '10 Opel Zafira 111 Anniversary
Thanks: 802
Thanked 685 Times in 439 Posts
Originally Posted by jakobnev View Post
I'll try to illustrate the difference with a couple of idealized graphs.
This is the idea that drives concepts like displacement on demand, valve throttling, and (to a lesser extent) variable valve timing. This is the idea that illustrates why a diesel engine is generally more efficient than an otherwise equivalent gasoline engine. The higher compression ratio of a diesel engine does not provide as much fuel economy savings as is generally thought.

It takes energy to generate a vacuum inside the intake manifold, and it takes even more energy to suck air past that hideously restrictive throttle plate. Those two energy draws are interrelated, but are in fact separate. That's in addition to having to suck air past the poppet valves in the head. Keep in mind that these energy draws are not used in generating any useful output.

It is still possible to generate an intake vacuum without using a throttle plate to restrict air. If you use a small enough venturi, you can allow the sonic speed of the incoming airflow to limit the airflow itself to act as an effective throttle, and the venturi will allow that self-limited airflow to expand to form the vacuum.

Last edited by t vago; 12-06-2010 at 08:44 AM..
  Reply With Quote