Re discussing an actual exam question - I claim immunity since I have not sat a test yet.

This has got me thinking. I figure the correct answer is 'no change', here is my reasoning...

For an auto waste gate - the pressure would be adjusted automatically - so no change.

For the fixed waste gate - at low altitude and high power the turbocharger would have little effect since the throttle would be set partially open only, so the leak would not be noticed.

For the manual and throttle controlled waste gate - the waste gate would be fully open and the exhaust would be completely bypassing the turbine.

Correct?

Doug

Hi all, (First post for me on this forum).

This question mentioned in post #7 has also got me scratching my head from my copy of issue 7, July 2011 CPL AGK.

During the compression and power strokes of a theoretical 4-stroke cycle, on a horizontally opposed engine -

a) the exhaust valve remains closed
b) both inlet valves remain open
c) both valves remain closed
c) both valves remain open

However, my book answer has answer 'c' as 'the inlet valve closes'

Without having the preferred choice of answer 'c' above (both valves remain closed), i chose answer 'a' as it's the best answer available to me assuming we're talking about the squeeze/bang cycle of the 'theoretical' 4-stroke,

Bob's answer to Q39 in my book is 'c' The inlet valve is open during the induction stroke and closes just after the compression stroke begins. This allows for gas to continue to flow through the inlet port due to inertia. It is called valve lag

Wouldn't this be the answer to an 'actual' 4-stroke cycle engine?

Hi Migraine,
I get your point. There seems to be a bit of ambiguity still with this question and whether it talks about a theoretical or actual 4-stroke cycle. We may need to tweak the question stem.

In the theoretical 4-stroke cycle, both valves will indeed be closed during the compression and power strokes (see pg 1.3. and 1.4 of the textbook). Therefore "(c) both valves remain closed" would be the answer.

The current answer options would seem to be talking about a real-life cycle where during the:
- Induction Stroke: both valves are open (valve overlap) then the exhaust valve closes
- Compression Stroke: the inlet valve stays open initially (valve lag) then closes, and the exhaust valve is already closed
- Power Stroke: both valves are closed then exhaust valve opens before BDC (valve lead)
- Exhaust stroke: the inlet valve is closed, and the exhaust valve is open but then the inlet valve opens so that both are open just before TDC (valve overlap)

So, in the compression and power strokes, in the actual engine it would be hard to pick one a correct answer out of the current options. Might be another errata on the way. I'll have a word with Bob.

Thanks for pointing it out.

Cheers,

Rich