Six Star Magazine Six Star Magazine Spring/Summer 2005 | Page 24
TECH TALK
power and grace
The new 6-cylinder horizontally-opposed SUBARU BOXER engine
I
t used to be that you had to choose: either you had a smooth
engine with perfect idle qualities and good fuel economy or you
had a powerful engine that gave you quick response and enough
power to pass with confidence. With the technology available today,
you don’t have to choose anymore. The proof is in the new H6
SUBARU BOXER engine found in the Outback 3.0R and 3.0R VDC
and under the hood of our new flagship – the Subaru B9 Tribeca.
It has been said that an engine is like an air pump; you need to
get air into it, mix it with the proper amount of fuel and ignite the
mixture at the right time to produce combustion and get things
moving. This might sound simple, but things need to happen
rather quickly. An engine at idle, say 700 revolutions per
minute (RPM) or 11.7 revolutions per second, needs to get air in
and out of the combustion chamber every 0.17th of a second
(it takes two revolutions to complete the process).
Minimum Overlap
Maximum Overlap
BDC
BDC
Intake
valve timing
TDC
Exhaust
valve timing
BDC
Intake
Valve lift
Small overlap
improves timing
This position offers excellent idle quality combined with improved fuel economy.
*European model shown.
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TDC
BDC
BDC
Intake
valve timing
TDC
BDC
Intake
Valve lift
Valve lift
TDC
Valve lift
Exhaust
valve timing
At 6500 RPM, that’s every 0.018th of a second. Much of the secret
is in getting the right amount of air at all engine speeds.
To get this done in the most efficient way, the new H6 uses two
technologies: variable cam timing and variable cam lift.
One of the first things technicians learn to recite in school and
the basis for how an engine works is “intake, power, compression
and exhaust.” As mentioned above, all this needs to happen at
engine speeds ranging from idle to the red line.
Variable cam timing allows us to vary the overlap between intake
and exhaust valves (varying the time they are both open at the
same time). This needs to be very little at idle for smooth operation
and good fuel economy but needs to be quite substantial at higher
RPMs to allow burned gases to be evacuated from the combustion
chamber and allow clean air and fuel to enter. The mechanism that
allows this to happen is controlled by the engine computer.
Overlap produces
Scavenging effect (due
to exhaust pulsation)
Inertia supercharging
effect produced by
advancing the intake
valve closing time
This position offers excellent combustion chamber scavenging to improve power output.