Starter Torque

[johnk]

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........

 

[BillyP]

Due to the more than twice the compression of the diesel engine verses gas, the diesel.

 

[johnk]

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

 

[jinman]

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.

 

[johnk]

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

 

[Nasty135]

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

 

[Ozarker]

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.

 

[uhmgawa]

</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.

 

[normde2001]

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.

 

[MMM]

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.

 

[Charlie_in_TX]

Where is the torque measured? Torque produces by the starter or on the crankshaft?

Lets apply a little math and see relitive pressure per square inch. A JD 4210 (28hp) has a displacement of 81 cu in. Thats 27 cu in per cylinder. Bore and stroke are not speced. If the bore is 3in the stroke will be 3.85in. Sounds about right for a low RPM hi torue motor. A 3in bore has a 7in surface area. At 400 psi compression that is 2800 lbs of force.

I will really guess on the 3.7l jeep. 3.7l is about 225 cu in or 37.6 cu in per cylinder. If the bore is 3.84in the stroke is 3.25in. Sounds about right for a high RPM motor. 3.84in bore has a 11.5 in surface area. At 175 psi compression that is 2025 lbs of force.

Now assuming the both have the same flywheel and starter the deisel will require more force(torque) for each compression stroke. However, the gas will require more work to be performed by the starter for each revolution of the crankshaft.

So in answer to your question.... it depends. Are we talking peak torque or average torque? Peak torque = 30hp deisel. Highest average = 3.7l gas.

 

[johnk]

I am just talking about a three second crank of the starter. Call it average time or whatever but the diesel will still require more torque. Even if it cranked for a minute it would still have to do the same amount of work in both cases. I never saw a car with two batteries in parallel unless it was an RV with exceptional draw for accessories.......Not for starting amperage. /forums/images/graemlins/grin.gif

 

[Henro]

Try the back door approach...

Pick two engines. Find the starter specs...whichever is larger/heavier/more copper mass/whatever should be an indicator of which engine requires more torque/power to turn it over. Especially if they are modern engines. Today engineers seem to build less fat into their systems...or should I say cushion/safety factor...

Symplistic approach...yes! But maybe just practical enough to settle the discussion.

And if that doesn't work, as suggested above, fall back on "father knows best..." /forums/images/graemlins/grin.gif

And if that doesn't satisfy, tell the kid to prove you wrong, since it will be a good learning experience. In writing naturally... /forums/images/graemlins/grin.gif

 

[Soundguy]

Though I don't have the technical specs on the starters on our heavy equipment in our yard.. I can say that the starter on every one of our diesels is larger than the starter on the gas jobs. We have one large stationary gas engine for a blower/burner motor.. big huge engine.. not sure what it is.. but has 4 barrel carb.. and looks to be as big / bigger than a 500.. still has a 'normal' looking car starter.

However.. the starter on our cat 3208 engine which powers a genset.. is so big that it is a struggle for me to lift it and tote it around... go figure.. We also have a cummins industrial engine on stationary pump.. not quite as big as the 3208.. but easilly the same size as the gas job.. and its starter is inbetween.. smaller than the cat.. larger than the gm's.. but like I say.. I don't have on the paper specs.. so i don't know exactly what CID size engines I'm comparing.

Soundguy

 

[Charlie_in_TX]

</font><font color="blue" class="small">( I am just talking about a three second crank of the starter. Call it average time or whatever but the diesel will still require more torque. Even if it cranked for a minute it would still have to do the same amount of work in both cases. I never saw a car with two batteries in parallel unless it was an RV with exceptional draw for accessories.......Not for starting amperage. /forums/images/graemlins/grin.gif )</font>
The average torque in the 3.7l engine will be more than that of a 30hp diesel. In other words, it will put more load on the battery.

On a second note you can't change the 'rules' 12 posts into the thread. Your origianal senerio was a 30hp deisel vs a 3.7l jeep V6. Changing the comarison to a 5.9l Cummings (or other LD truck deisel engine) is not the same as your original senerio.

So what is it? Do you want to compare the 30hp to the 3.7l or someother comparison?

 

[johnk]

To get back on track the 30 HP 3 cylinder with the gas 3.7L. All the evidence so far clearly shows the diesel takes more torque to start. Thats what I'm getting out of this in a round about sort of way with everyone stating different facts. Bottom LINE. Diesel takes more torque or power to start on a three or 4 second crank of the starter. /forums/images/graemlins/confused.gif

 

[bgott]

The easy way to find out would be to look up the specs on each starter motor and compare the amp draws.

 

[nomad]

</font><font color="blue" class="small">( Diesel takes more torque or power (than Gasoline) to start ..)</font>

I added "than gasoline" to generalize your question because I think what your main question isn't about the figures/numbers, but a general comparison of required starting powers by "virtually identical" diesel and gasoline engines. Gasoline engine will require more power than the diesel. Why? Because reactional force against the piston during the compression process in the gasoline is higher than that of diesel. Because only air is being compressed in the diesel while in the gasoline a mixture of air and evoporized fuel is being compressed. Mizture of fuel+air has much higher theormodynamic pressure than the air in the diesel. But starting electric motors of diesel engines usually have higher powers than that of gasolines. This is because the diesel engines are heavier mechanics and in case of unsuccessfull flaming attempt at first strokes in the diesel, electric motor should still help the crankshaft turn and if this happens, mixture of diesel fuel + air will be higher pressure than the gasoline mixture.

 

[R_squared]

Here are a series of three, starter current waveforms that I captured this weekend. They are from two gas burners and one diesel engine. The waveforms are for relative comparisons only!!! To convert the electrical energy (Kw/time) to an instantaneous mechanical force (Foot-pounds/time) would probably be possible, but there would be too many factors and losses to consider for the results to be accurate. The equipment used was a Fluke 199 ScopeMeter and a F.W.Bell UM-7700 current clamp. The current clamp is a Hall Effect type and can measure up to 1000 AC or DC amps. Each curve was captured until the engine started and the solenoid disengaged. The vehicle batteries were in good condition and as you can see, it did not take long for each engine to start in this warm weather.

The first waveform is from a 1989 F150, 6 cylinder, 4900cc engine. The compression ratio is approximately 9:1. The inrush current exceeds 1000 amps, but only for 20 or 30 milliseconds.

 

[R_squared]

The second waveform is from a 2002 Kioti DK45, 4 cylinder, 2197cc engine. The compression ratio is approximately 19:1. The inrush current equals 1000 amps, but only for 50 milliseconds. The glow plugs are drawing approximately 80 amps - this is shown to the left of the inrush peak. After the engine starts the current draw returns to near zero. The little diesel engine definitely did take more current to start - the area under the curve is greater.