Engineering multiple choice from hell

Factual Questions
1

Lets suppose I have a diesel engine on a research bench.
This is a ‘magic’ engine with two special qualities:

  1. It can use any oil for lubrication.
  2. It can use any oil for fuel.

Using various on-board emission sensors which can monitor HCs, NOx, CO, CO2, O2, and soot, the on-board computer can throttle back the fuel to keep the exhaust clean.

One day I have the idea to tap directly into the sump to use the lubrication as fuel, thus eliminating the need for regular oil changes. A side benefit is the lubrication system is continuously clean. Every drop burned in the engine is replaced by fresh fuel from the fuel tank.

Now, when the engine is first fired, it is using very dirty oil (contaminated by blow-by gasses).
NOTE: I am using a ‘magic’ lube/fuel filter which removes the microscopic particles which would otherwise clog the injectors.

In order for the engine to run smoke-free, the engine throttles itself back to only about 15% full throttle…this is so all the extra oxygen in the intake charge burns up all the dirty oil. New, fresh, clean oil replaces every drop of dirty oil at the same rate it is used up. Themore the engine runs, the more dirty oil is replaced by clean oil, and thus the engine can start running at a higher percentage of full throttle…which of course replaces dirty oil at a faster rate.

Suppose we were to measure certain critical time intervals at specific throttle percentages:

20%=1/5
interval 1
25%=1/4
interval 2
33.33%= 1/3
interval 3
50%=1/2
interval 4
100%=full throttle

Question: Does

a. interval1>interval2>interval3>interval4
b. interval1<interval2<interval3<interval4
c. interval1=interval2=interval3=interval4
or
d. other

2

Wouldn’t the time intervals decrease as the the throttle is opened more and more, thus increasing engine speed? So, it would be b, unless I’m misunderstanding what you mean by “time interval”.

3

QED is a frickin’ genius.

My eyes crossed somwhere in the middle of the first paragraph.

4

So did mine, 'cause I should have said a. :smack:

5

Going from 20% to 25% is 5%. From 25% to 33% is 8%. 8% being larger than 5% would make interval 3 larger than interval 2. But the ramp up of the engine would make interval 2 larger than interval 3. So the question is which of those two factors would be larger. Unless you can quantify how quickly the throttle is opening up, I don’t think you can answer this question.

6

Answer - Do an oil change before starting up, and it will all be clean therefore you go straight to 100%.

7

Maybe this will clarify things some:

The engine displacement is 5.0 liters.
The sump capacity is 4 liters.
The stoichiometric ratio of the lube/fuel oil is 20/1.