Very Basic Engine Question

[Ducked]

Torturing my Technology English class with a MidTerm mostly on the IC engine.

I've given them this URL as reference for preparation, and will base some of the questions (probably mostly vocab gap fill) on a reworking of it .

http://www.fesconstruction.co.uk/KS3/Cams/stages_of_engine.html

However, I've just noticed that the Intake Stroke diagram (top one in the set) seems wrong, or at least misleading, since the piston is moving upward.



If the intake valve was open at the point shown that would push some mixture back up the intake path, though this movement would be quickly reversed.

Does this actually ever happen?

If it IS actually wrong I might put it in as a "critical thinking" question, probably just for bonus points since these students aren't very big on critical, or indeed any, thinking.

 

[eyeofthetiger]

Maybe it's one of those silly Miller-cycle engines that pushes half of the intake charge out in order to decrease the compression ratio. That picture is really bad, though.

 

[Linctex]

The connecting rod needs to be on the OTHER side, same as the "power stroke" picture

 

[bdcardinal]

On a higher performance engine with proper camshaft overlap, the piston will still be traveling up when the intake valve opens. They use the velocity of the remnants of combustion being forced out the exhaust valve to help draw in the intake charge.

 

[SZR48207]

They screwed up on the crankshaft arrows and positions but the top half is OK.

I would not expect your class to know timing well enough to get you on a technical error, but as a student I would challenge anything less than a perfect score for this particular question.

 

[Ducked]

Originally Posted By: Reddy45
They screwed up on the crankshaft arrows and positions but the top half is OK.

I would not expect your class to know timing well enough to get you on a technical error, but as a student I would challenge anything less than a perfect score for this particular question.


Not using the question as on the website. I'll give them a (reworded) description of the 4 strokes with gaps in it, and a wordlist with which to fill the gaps.

What I'm wondering is if the top diagram is broken enough to work as a critical thinking question, along the lines of

"IV Critical Thinking: The Intake stroke diagram (top one of the four) is wrong. Write a stand-alone sentence here saying what is wrong with it."

Strictly speaking valve-overlap might mean it isn't necessarily broken, as I suspected but have now had confirmed here, thanks to the poster above. However, it doesn't seem to be doing what he describes, since the exhaust valve is shown closed so couldn't be involved.

My students certainly won't know about valve overlap, and I'd bet whoever drew that diagram (not ungrateful, saved me doing it myself) didn't either, so maybe it would still be a legit question.

I often use bollocks in textbooks as the critical thinking element in my courses. Never any shortage of material.

Taiwanese students hate it

 

[Silk]

A single cylinder engine is pictured, very possibly a BSA B34 Goldstar by the look of it. In which case the inlet valve opens 65 degrees before top dead centre.

 

[Ducked]

Originally Posted By: Silk
A single cylinder engine is pictured, very possibly a BSA B34 Goldstar by the look of it. In which case the inlet valve opens 65 degrees before top dead centre.


Could be. Or it could be a section through an in-line 6. No way to tell.

65 degrees BTDC does tend to undermine the CT thing a bit though.

What is the exhaust valve on a BSA B34 Goldstar doing at that point in the cycle?

 

[ZeeOSix]

Yeah, it's wrong as others have already pointed out. These are super basic 4-stroke IC engine diagrams, so the top diagram isn't going to be describing some rare 4-stroke engine.

 

[Silk]

Originally Posted By: Ducked
What is the exhaust valve on a BSA B34 Goldstar doing at that point in the cycle?


65 85 80 55

 

[SR5]

I think it's supposed to be a basic illustration of the simple concepts of a 4 stroke engine, to me this makes the first picture wrong.

Remember: Suck, Squeeze, Bang, Blow

 

[Silk]

Ooops, I think it's a Honda, I'm not sure how I got that confused with a BSA.

 

[Rick in PA]

The diagram is wrong.

To make the diagram right you'd have edit the first illustration:
1. Mirror the image of the crankshaft and connecting rod to the left side.
2. Leave the arrow where it is.

As a side note, I'd say what they label as compression is actually ignition, but I'll let them slide on that.

 

[d00df00d]

Originally Posted By: bdcardinal
On a higher performance engine with proper camshaft overlap, the piston will still be traveling up when the intake valve opens. They use the velocity of the remnants of combustion being forced out the exhaust valve to help draw in the intake charge.

Excellent point -- but if this were depicting overlap, shouldn't the exhaust valve be open in the pic as well?

Also, how many engines have their intake valves open at ~45º BTDC?

EDIT: Beaten to it.

 

[d00df00d]

Originally Posted By: Ducked
What I'm wondering is if the top diagram is broken enough to work as a critical thinking question, along the lines of

"IV Critical Thinking: The Intake stroke diagram (top one of the four) is wrong. Write a stand-alone sentence here saying what is wrong with it."

I'd say so. Seems like a good call. Maybe make it extra credit?

 

[Rick in PA]

Second problem with the diagram.

The illustration labeled compression is actually ignition. Ignition is the start of the power stroke.

It's just a poor diagram. I wouldn't use it.

 

[Ducked]

Originally Posted By: Silk
Originally Posted By: Ducked
What is the exhaust valve on a BSA B34 Goldstar doing at that point in the cycle?


65 85 80 55


IIRC that'd be with the 65-2442 and 65-2446 intake and exhaust cams respectively.

I'm impressed that you can identify the cams in use from that rather basic diagram.

Go to the top of the class, and have a Red Smartie.

 

[Ducked]

Originally Posted By: Silk
Ooops, I think it's a Honda, I'm not sure how I got that confused with a BSA.


If you deliberately disrupt my class I'll kick you out, and you won't be able to graduate.

 

[Ducked]

Originally Posted By: Rick in PA
Second problem with the diagram.

The illustration labeled compression is actually ignition. Ignition is the start of the power stroke.

It's just a poor diagram. I wouldn't use it.


You mean the second picture down? No spark, so I'd say it isn't ignition. Ignition and compression obviously arent incompatible anyway.

I don't have to use this in the test but I've already posted it as a revision aid, so its too late to undo that.

Perhaps there shouldn't be a spark still visible in the power stroke picture (3rd one down) but that's getting a bit ultra-picky. The human retina retains the image of a spark for longer than its duration anyway, if you could see it which of course you can't unless you'd got a glass engine.

 

[cjcride]

Originally Posted By: bdcardinal
On a higher performance engine with proper camshaft overlap, the piston will still be traveling up when the intake valve opens. They use the velocity of the remnants of combustion being forced out the exhaust valve to help draw in the intake charge.

^This