Date: Mon, 19 Jul 1999 17:02:22 -0400
Reply-To: Martin Jagersand <jag@CS.YALE.EDU>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Martin Jagersand <jag@CS.YALE.EDU>
Subject: Westy brainteaser answer (Limbo campout.)
Here are my suggestions to answers. You might want to check the other
posted answers as well. They are much more entertaining.
The first question was to compare the two Diesel engine types -
direct (DI) and indirect (IDI) injection. A Diesel, as opposed to a
gas engine, always runs with an excess of air. How much of the air can
be burnt depends on how well the fuel can be mixed with the air. If the
fuel doesn't mix well, over rich conditions in part of the cylinder
will cause soot formation, while in other parts unburnt oxygen remain.
A typical direct injection engine can burn up to about 50% of the
avaiable oxygen in the air. The swirl (or prechamber) designs commonly
called indirect injection was developed to increase this ratio, and also
to make the burn more even to reduce noise and increase the rpm
range. An indirect engine can typically burn about 70% of the available
oxygen. The drawback of the IDI design is that because of the larger total
area of the cylinder walls and the precombustion chamber more energy
is lost as waste heat than in the DI engine.
With the figures given the DI can only burn about 0.50/0.70 = 0.71
as much of the ar as the IDI, but because the uses the air and
fuel more efficiently it can generate 0.50/0.70*250/204 = 0.87
of the power.
In pratice the more modern TDI engine has a computer controlled
fuel control which can set the max fuel metering more exactly,
and gains some extra power from that. Hence the power from a
DI and IDI, assumed otherwise identical, is about the same.
This is a very simplified reasoning of course. But it serves to
point out the fact that while the DI engine is generally more
fuel efficient than the IDI, they are not necessarily more powerful.
Question 2:
===========
How to go through a tunnel with 4 busses using 1 light, and at
most 2 busses in the tunnel at one time.
The speeds of the busses are 1, 2, 5 and 10 min respectively.
In a naive approach the fastest always goes back:
(1,2),(1),(1,5),(1),(1,10) for a total of 19min
Where (1,2) means the 1 and 2 minute busses travel together with the
light in 2 min. (1) means the 1 min bus goes back alone etc.
However we can gain two minutes by this sequence:
(1,2),(1),(5,10),(2),(1,2) total 17min
/Martin
--
Westy 1.9l Turbo Diesel
Quantum 1.6l Turbo Diesel
New and used parts for sale, gas and Diesel:
http://www.cs.yale.edu/~jag/vw/forsale.html
Martin Jagersand email: jag@cs.yale.edu
Computer Science Department jag@cs.rochester.edu
Yale University
Slow down and visit the VW diesel Westy page:
WWW: http://www.cs.rochester.edu/u/jag/vw
-------------------------------------------------------------------
