Discovering the Power of a 1.4L Car Engine's Starter Motor: An Equation Guide

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Discussion Overview

The discussion revolves around determining the power of a starter motor for a 1.4L car engine, specifically focusing on the equations and factors involved in calculating this power. Participants explore concepts related to torque, resistance during engine startup, and the mechanics of internal combustion engines.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants inquire about the equation used to calculate the power of the starter motor, specifically referencing the power value of 17.98 watts.
  • There is a suggestion to consider torque rather than power initially when analyzing the starter motor's function.
  • Participants discuss the resistance encountered when starting the engine, including factors like cold engine oil and atmospheric temperature.
  • One participant proposes that crank resistance through the crankshaft is a significant factor in the resistance felt during startup.
  • Discussion includes the four parts of the 4-stroke engine cycle and what occurs during engine cranking, with participants identifying which strokes are present or absent during startup.
  • There is a clarification that the resistance during compression is due to closed valves, which contributes to the overall resistance to turning the crankshaft.
  • One participant expresses skepticism about the power value of 17.98 watts, suggesting it might be a misprint for 17.98 kilowatts or 1798 watts, indicating a potential misunderstanding of the power requirements.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct power value for the starter motor, and there are multiple competing views regarding the factors contributing to resistance during engine startup.

Contextual Notes

Participants express uncertainty regarding the exact values needed for calculations, particularly the resistance due to cold engine oil and the specifics of the starter motor's power requirements.

elephunk
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Basically using the engine size, the power of the starter motor was found to be be 17.98w. The cars engine is a 1.4L. What's the equation used to find the power of this starter motor?

Basically I need to find how they got this so I can move onto my next part of the coursework.

Would be grateful for any assistance.

Thanks,
 
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elephunk said:
Basically using the engine size, the power of the starter motor was found to be be 17.98w. The cars engine is a 1.4L. What's the equation used to find the power of this starter motor?

Basically I need to find how they got this so I can move onto my next part of the coursework.

Would be grateful for any assistance.

Thanks,

What are your thoughts on this? Hint -- think initially in terms of torque, rather than power.
 
Am I supposed to be looking at the torque equation?

torque = r x F??
 
elephunk said:
Am I supposed to be looking at the torque equation?

torque = r x F??

Partly. What resists the turning of the crankshaft of the motor while you are trying to start it? You need to overcome that resistance to turning, correct?
 
The engine oil. Its cold and not up to temperature.

It seems like the atmospheric temperature is meant to be -20 for this assignment. Yes, I have to overcome that resistance. How do I overcome it though if I don't have a value for it?

Sorry for being so confused!
 
elephunk said:
The engine oil. Its cold and not up to temperature.

It seems like the atmospheric temperature is meant to be -20 for this assignment. Yes, I have to overcome that resistance. How do I overcome it though if I don't have a value for it?

Sorry for being so confused!

No, that's a small part of it. Think about pull-starting a lawn mower, or kick-starting a dirtbike. What causes all that resistance that you feel?

Are you pretty familiar with how internal combustion piston engines work? There's a pretty good intro article at HowStuffWorks.com:

http://auto.howstuffworks.com/engine1.htm

The animation of the 4-stroke engine cycle is pretty helpful.
 
I'm pretty familiar with them but can't think of what this resistance could be. I'm thinking its crank resistance through the crankshaft. Is that correct? I've never pull started a mower (all electric here) or kick started a dirtbike :(

I haven't got two deep down with cars either :(
 
From the engine web page that I linked to, what are the 4 parts of the 4-stroke engine cycle? If the engine is being cranked trying to start, what is missing so far from those 4 parts of the cycle? But what is still present?
 
Only intake and combustion will be present when the engine is trying to start. Correct?
 
  • #10
elephunk said:
Only intake and combustion will be present when the engine is trying to start. Correct?

No, combustion is the thing that's not happening yet. What are the other parts of the cycle?

(I have to bail for a few hours. Will try to check back later tonight.)
 
  • #11
Sorry, I mean intake and compression (its late here :P )
 
  • #12
elephunk said:
Sorry, I mean intake and compression (its late here :P )

No worries. There's also the exhaust stroke, where the (unburnt during starting) mixture is blown out the exhaust valve. So on intake the intake valve is open, and on exhaust the exhaust valve is open. What valves are open on compression? What does that mean about the resistance you might feel on the crankshaft during these three parts of the 4-stroke starting cycle?
 
  • #13
On compression they are closed. Is this the resistance??

Thanks,
 
  • #14
elephunk said:
On compression they are closed. Is this the resistance??

Thanks,

Yes, good. The resistance to turning the crankshaft during starting is from the compression of the cylinders. Now you need to calculate what that resistance is, and convert it into the starting torque required. That will be how you size the starter electric motor.
 
  • #15
That 17.98 watts seems suspiciously low.

Maybe 17.98 KW? That's about 24 HP which could be reasonable, but maybe a bit high.

Perhaps 1798 watts. 12 V at 150 amps is in the right area. Maybe that decimal point was just not meant to be there.
 

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