Engine HP ratings?

[EDL]

I have about 15 acres, not all of it mowed, of course, but with my current old Craftsman 18HP 42" cut rider the area I do mow is a good 6-7 hours of straight mowing. I'm sick of basically doing nothing but mowing every weekend from May through October, so I've decided to take the plunge next year and drop way more on a mower than I ever thought I would. I'm looking at the BadBoy XP with 72" deck and the 36 HP Vanguard engine. I'd love the diesel, but no way I'm dropping $15,000+ on a mower, lol.

Anyway, looking around at all the various brands of zero turn mower they all basically use the same engines and as I look at the specs I started scratching my head....specifically the claimed HP doesn't add up with the listed torque and RPM numbers. I know from building enough engines in my hotrod days that peak torque is achieved before peak HP in most engines, so I would expect the peak torque rating to show slightly less than peak HP, but the claimed specs by these engine manufacturers vary quite wildly.

For example, the Kawasaki FX751V claims 27HP and 44.6lb-ft of torque at 2,400 RPM. Well, if we follow the formula for HP (since it's a calculated figure), 44.6lb-ft at 2,400 RPM is only 20.4HP. That's a tad over 6.5HP less than claimed. The only way to get 27HP would have to be at a fair bit higher RPM.

If that isn't weird enough, the 4-cyl Cat diesel used in these ZTR's is rated at 35HP, yet they claim 94lb-ft of torque at 3,000 RPM. Do the formula on that and 94lb-ft at 3,000RPM is 53.69 HP!

Seems the claimed numbers don't add up.

 

[exotion]

Not sure about all that. What's the formula you used? Could a single cylindar engine formula be.different than a multi cylinder engine?

 

[panabiker]

The specs for HP and for torque are usually not at the same RPM. You calculated the HP value from the torque value at 2400 rpm. Could the HP number be specified at a higher RPM? For the 4-Cyl diesel, max torque is usually reached at 1500-2000 rpm, so maybe the HP is spec'ed at 3000rpm.

 

[EDL]

Not sure about all that. What's the formula you used? Could a single cylindar engine formula be.different than a multi cylinder engine?

HP is calculated as torque times RPM divided by 5252.

A dyno measures the torque and RPM curve and the computers calculate the HP.

Once an engine achieves peak torque, as the revs continue up the torque begins to drop. Peak HP is the point on the curve where the torque drop rate equals the RPM rise rate. Anything past that and the HP figure drops.

HP is a function of time. An indication of an engines ability to do work. Cylinder configurations (or even something other than a reciprocating engine, such as a turbine) make no difference. HP is HP.

 

[EDL]

The specs for HP and for torque are usually not at the same RPM. You calculated the HP value from the torque value at 2400 rpm. Could the HP number be specified at a higher RPM? For the 4-Cyl diesel, max torque is usually reached at 1500-2000 rpm, so maybe the HP is spec'ed at 3000rpm.

Yes, peak HP in a typical gas engine is almost always at a higher RPM than peak torque, but the difference in HP at the given RPM's seems a bit high. In other words, what I'm getting at is that claimed HP rating on those gas engines seems a bit...optimistic.

In the case of the diesel, they give the peak torque at 3,000 RPM (94 lb-ft). At peak torque that's 53 HP, a fair bit over the peak rated HP. That doesn't make sense. I have never seen an engine that puts out 1.5 times the peak HP rating...they would have labelled it as a 53HP engine.

Diesel peak torque RPM varies a lot depending on the displacement (smaller diesels typically will run at higher RPM than large displacement ones). The 5.9L Cummins in my Ram 2500 puts out peak torque at 1600 RPM, and engine RPM redline is 2200 RPM. Large diesels, like those in an 18-wheeler typically make peak torque around 1100 RPM with an RPM redline around 1500 RPM. Diesels are RPM limited by design, due to the pressures, thus they make up for the lack of RPM with massive torque output. In the end though all that torque gets traded back to RPM in the drive train in order to spin the wheels at usable speeds.

I dunno, it appears to me some small engine makers are making some pretty inflated claims, and at least one diesel maker isn't making any sense....at least mathematically.

 

[djdicetn]

Yes, peak HP in a typical gas engine is almost always at a higher RPM than peak torque, but the difference in HP at the given RPM's seems a bit high. In other words, what I'm getting at is that claimed HP rating on those gas engines seems a bit...optimistic.

In the case of the diesel, they give the peak torque at 3,000 RPM (94 lb-ft). At peak torque that's 53 HP, a fair bit over the peak rated HP. That doesn't make sense. I have never seen an engine that puts out 1.5 times the peak HP rating...they would have labelled it as a 53HP engine.

Diesel peak torque RPM varies a lot depending on the displacement (smaller diesels typically will run at higher RPM than large displacement ones). The 5.9L Cummins in my Ram 2500 puts out peak torque at 1600 RPM, and engine RPM redline is 2200 RPM. Large diesels, like those in an 18-wheeler typically make peak torque around 1100 RPM with an RPM redline around 1500 RPM. Diesels are RPM limited by design, due to the pressures, thus they make up for the lack of RPM with massive torque output. In the end though all that torque gets traded back to RPM in the drive train in order to spin the wheels at usable speeds.

I dunno, it appears to me some small engine makers are making some pretty inflated claims, and at least one diesel maker isn't making any sense....at least mathematically.

Most likely the recent class-action lawsuit that small engine manufacturers had to deal with because of "over-inflated" HP ratings changed the calculation of HP. It is my understanding that you want to look at the torque rating to determine the cutting power of mowers. Are you certain that the HP calculation for small air-cooled engines is identical to the calculation for "hot rod" engines?????

 

[panabiker]

The calculation of HP from torque and rpm does not have anything to do with the type of engines. It was derived in physics before the invention of internal combustion engines. Power = torque x angular velocity. That is, 1 watt = 1 Newton-meter x 1 radian per second. The 5252 factor is to accommodate the different units

 

[djdicetn]

The calculation of HP from torque and rpm does not have anything to do with the type of engines. It was derived in physics before the invention of internal combustion engines. Power = torque x angular velocity. That is, 1 watt = 1 Newton-meter x 1 radian per second. The 5252 factor is to accommodate the different units

Soooooo, what formula were the manufacturers using that led to the lawsuit over inappropriate HP ratings????
From what I understand it made all engines' hp specs for the same engine a lower hp rating than before. If the calculation hasn't changed how do you explain the universal adjustment?

 

[panabiker]

One trivial aspect of change, believe or not, is how you can advertise the horse power. For example, on an engine rating of 10 hp, test results were required to show the engine put out 8.5 hp under the old standard. With the new SAE J2723 standard, a 10 hp engine must be tested to 98% of the advertised power, that is, 9.8 hp.
Then there is a methodology on how and under what conditions the torque and rpm curves are obtained, which then determines the peak HP.

 

[djdicetn]

One trivial aspect of change, believe or not, is how you can advertise the horse power. For example, on an engine rating of 10 hp, test results were required to show the engine put out 8.5 hp under the old standard. With the new SAE J2723 standard, a 10 hp engine must be tested to 98% of the advertised power, that is, 9.8 hp.
Then there is a methodology on how and under what conditions the torque and rpm curves are obtained, which then determines the peak HP.

Thanks for the follow-up info!!!! I'm definitely not an engineer but that all makes sense to me:0)
That's what I like about this forum.....if you don't understand something there will be a user that does and is willing to share their knowledge. I learn something new almost every time I'm on here. Thanks!!!!