PT Model Comparison

[marrt]

I have attached a small spreadsheet I created to compare the different PT models. Specifically, I wanted to compare HP and Torque ratings for each tractor, by the weight of the tractor. I was trying to get an understanding of how powerful a given tractor would be for its weight. For example, a tractor with the lowest weight per ft/lb of torque should, hypothetically, perform better in a give activity such as mowing on hills. Using this theory (which may be flawed…see below), the 425 would be the most powerful tractor, relatively speaking. The 1845 would be next.

That said, there are several problems with this approach, such as:

- torque ratings for Duetz are converted from nm to ft/lbs. Not sure the measurement methodology is the same.
- I assumed PT still uses the L 1011 Deutz engine family and not the L 2011
- torque ratings are not measured at the maximum HP output but at maximum torque. Therefore, real world behavior may be different.
- the torque rating for wheel motors was not considered
- the diameter of the tires was not considered
- the weight of the mower and the amount of power needed to drive the mower motor was not considered
- etc…..

 

[Bob999]

I like the table and wish Power Trac would put more information like this on the website. However, you have to be careful in drawing inferences from the data. I believe that a measure of a tractor is the work it is able to do. The measurements in the chart relate to the ability of the various models to move themselves about. However, in a typical working situation there is an operator in the seat, and if we assume 200 pounds for the operator, that amounts to nearly 15% of the weight of a 400 class tractor, but only about 5-6% of the 1800 series. It might be appropriate to adjust the figures for the operator.
Secondly, attachments like a mower take a large amount of power. The 400 series tractors use 48-60 inch mowers while the 1800 series use 72 in mowers. Assuming that the power needed for a mower is proportional to its size, the power demand of the mower does not increase proportionately for each class of tractor-- 48"-72" is a 50% increase while 50"-72" is only a 25% increase. The power increase from a 400 series to a 1800 series is 22-25 hp to 45 hp, an increase of 80 to 100%-and thus the larger series has a larger percentage of its power available to move itself, and its operator, around while operating a mower.

 

[Schultz]

I think engine "peak" HP & Torque values can be very misleading. A better number to use is the continuous hp & torque values. Unfortunately, not all manufactures provide this info on their web sites, and some may be more conservative than others about these continuous values. As you are likely aware not all 22 or 25 or 30 hp engines are created equal, some have brooder power bands than others, especially diesels, but the peak values may not show this. This is where torque rise from maximum hp comes into play. If an engine has high torque rise then it's able to generate nearly the same hp over more rpm. Look at the power curves for Kawasaki 23hp Kawasaki 23hp compared to the B&S 23 hp B&S 23hp (I'd use kohler I couldn't find this information). The Kaw when heavily worked at 3600 rpm won't loose rpm's or bog down like the B&S.

If you really want to know how much available power there is, the area under the hp/rpm curve will give you a total available power number, but none of the manufactures provide this information. This would be great then all we would have to look at is one number to determine if one engine was more powerful than the next @ the same rated HP. A crude estimation of the area under these power curves for B&S 23hp engine would be 30,200 and 33,150 for the Kaw, about 10% more total power. Then if you compare this to a Kubota diesel of the same HP the difference is even greater.

 

[wasabi]

Hey, Marrt. I'm feeling left out. How come you don't have figures for the 2445 on your chart? /forums/images/graemlins/confused.gif

 

[johara1]

you said it all!!!! this is the area that counts! this why they changed the wheel motors on the 425 to counter small power curve of the engine.the higher torque motors take less power.this will be very noticable. /forums/images/graemlins/cool.gif

 

[marrt]

I'm still not sure how useful this chart is....however, I will add a couple additional models tonight.

 

[marrt]

Ok, I finally added a couple models to the comparison spreadsheet. I also sorted the tractors by “pounds tractor weight for one ft-lb of torque.” The result is that the 1850 comes out the “hoss” of the bunch. This is not a surprise I guess. Also, I converted kw to hp. Therefore, some tractors have slightly less hp less than PT reports.

 

[marrt]

Couple additional comments on the above chart:

- There are likely significant differences between the ISO and SAE standards for intermittent and continuous hp and torque measurements
- Even though the 425, for example, comes out strong in the rating, this is with no load on the engine. Once a load is applied, a mower for example, then substantially less power is available for the tractor. To Bob999’s point above, the addition of a load to the tractor will dramatically change the ratings.

I did a small side calculation on the impact of adding a mower to the 425 and 1845, assuming the mower will need 11 hp and 15 hp respectively. The result is that the 1845 has significantly less pounds tractor weight per ft-lb of torque, compared to the 425, when a mower is attached. For example, without a mower attached, the 425 and 1845 would probably climb a given hill about equally in my estimation. However, with a mower running, the 1845 would have a big advantage.

 

[Anonymous Poster]

I would add a couple more points - we also have to take into consideration the type of pump and wheel motors
and how well they perform (ie actual ouput to the wheels) and also the gearing of the machine - for example - i know that the 1460 is geared much lower than the 1845 which give a lot much pushing power for snow (of course the weight helps as well) - from what I understand from Richard who sold me his 1845 in favor of the 1460 - the 1460 is really a beast - yet it comes out way down on the chart - in torque to pounds - I think it is geared 1/3 lower which would probably bring it in comparable or better than the others

Also - I would have to take issue with the 425 vs 1845 climb ability - because i have already had mine out and compared the two since the snow has finally melted - (I still have the 425 and hope to be delivering it next week ) the 1845 climbs the steep (21degree) hills easier

My feeling is that the 1845 more effectively and efficiently uses all the power and torque available from the engine - My guess is that it simply has higher quality (and more expensive wheel motors)
that do a better job putting the torque to the ground - I have not actually examined and compared the specs of each wheel motor because we have not nailed down actual model numbers - but this is my opinion in side by side testing of the machines on a 21 degree slope --- with the mower running (which I have not yet tested) the 1845 will be even better - because the mower will take up a greater amount of comparable engine torque in the 425 (at least with the 60 inch)
I think this is why PT went to heavy duty wheel motor in the 425.

Oh and one other thing - the 1845 with single turfs is gear higher than the 425 as well ( again I side by side tested) - which again should but does not give it less climb than the 425 - with dual turfs the 1845 is geared down (23 inch tires) that gives it the 40 degree slope abiltiy that they advertise.

Anyway - thought I would throw out my two cents after comparison -

 

[Anonymous Poster]

Oh by the way - I know that you pointed all this out in your opening remarks - ie ignore wheel motors tire size etc

just thought that in side by side test - I noticed that these factors make a big difference - maybe we can incorporate these figures into the analysis - would have to use manufacturers spec - which as you already point out may or may not represent actual field output -