Starter Torque

   / Starter Torque #1  

johnk

Elite Member
Joined
Jun 7, 2003
Messages
2,649
Location
western NY
Tractor
Kubota GST Grand L3130 w/ 723 loader, Ags
My son and I got in a discussion about what is harder to crank over. A diesel of 30 HP or a 6 cylinder car gasoline engine. Would it take more torque to turn over the 22:1 30 HP diesel or the 7.5 to 1 gas engine. I figured it would take a higher torque starter for the diesel and my son says no. Who is right and why???????Thanks for any info to helpsettle a discussion...........


Making misstakes is learning. Not making them again is knowledge........
 
   / Starter Torque #2  
Due to the more than twice the compression of the diesel engine verses gas, the diesel.
 
   / Starter Torque
  • Thread Starter
#3  
He says with the gasoline engine and more cylinders there is more mass to get in motion and therefore more torque needed for the higher displacement gas engine......... /forums/images/graemlins/blush.gif
 
   / Starter Torque #4  
John, I think you are going to have to add a lot more specifications for both engines. For example: What are the relative size of the pistons? What is the size of the flywheel (starter motor to flywheel gear ratio). I think the starter/flywheel gearing is designed to show equal torque to either starter.

Now my diesel truck has two batteries, but it also has high current air preheaters, not to mention that it's a 6-cylinder, 5.9 liter engine with 16:1 compression when the turbo boost is zero. All that is plenty for two batteries to handle.
 
   / Starter Torque
  • Thread Starter
#5  
I was just speaking in general terms. A 35 HP tractor diesel and say a 6 cylinder 3.7 liter Jeep engine. What would take more torque to crank over? Generally speaking..........I say the diesel.... /forums/images/graemlins/smile.gif
 
   / Starter Torque #6  
The diesel requires more torque to over come the compression stroke, the gas engines lower compression can be turned over much easier.Unless your talking about a 13:1 Bigblock thats just run the quarter and is a bit warm and tight that will take considerably more torque to restart,which would be closer to the diesel at that point .The starter for a stock dodge cummins puts out about 4hp no car starter puts out that much, /forums/images/graemlins/wink.gifDads are always right! GEE I hope my dad doesn't read this!! /forums/images/graemlins/tongue.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/grin.gif
 
   / Starter Torque #7  
He is wrong on two counts. First, it is unlikely that the mass of that 6 cy gas engine is greater than the mass of the 30hp diesel (probably 4 cyl). The pistons, rods and crank of the diesel are much beefier and heavier that the pistons, rods and crank on that gasser.

Two, it really doesn't matter that there are 6 cylinders. What matters is how many of those pistons are on the compression stroke at any one time. At best, one is near the top of the compression stroke and one is on the way up. Of the other 4, two are going down and two are on the exhaust stroke.
 
   / Starter Torque #8  
</font><font color="blue" class="small">( My son and I got in a discussion about what is harder to crank over. A diesel of 30 HP or a 6 cylinder car gasoline engine. Would it take more torque to turn over the 22:1 30 HP diesel or the 7.5 to 1 gas engine.)</font>

This is quite complex and depends upon many factors
including:

- displacement of the engine
- number of cylinders
- where each piston is located within its cycle
- cylinder compression ratio
- mass of cylinders, linkage, and camshaft
- aspiration mechanism (eg: gasoline vs. diesel)
- minimal self-sustaining speed of the engine

Note it is assumed the engine is at rest and needs
to overcome resistance of the compression stroke
as well as vacuum of (what would normally be) the
combustion stroke. Add to this the partial
vacuum resistance of the intake stroke for a
gasoline engine.

All of this ignores frictional losses which vary due to oil
temperature; valve restriction for intake/exhaust strokes;
fuel injection pump loading; outboard mechanical loads,
etc.. I'd expect approximate mathematical models to
exist for the truly curious. However it isn't as simplistic
as gasoline vs. diesel.
 
   / Starter Torque #9  
I think it's basically going to depend on how many square inches of piston area are under how much compression at any one time.
Back in the days before motorcycles had electric starters on them, 500cc, (30.5cid), single cylinder engines and 74cid twin Harleys were so hard to kick start they were equipped with compression release levers, wheras 650cc, 40cid, twin engines, with same compression ratio, did not need them.
 
   / Starter Torque #10  
Definitely diesel,, which must come to combustion speed as there is no spark to start combustion. A tuned gasser should start at the touch of the key, where the diesel must overcome repeated high compression strokes before ignition occurs. I service large 16cyl CATs with air starters that simply will not cold start unless spinning 400rpms or more.
 
 
Top