I should think it would be self explanatory. Quality of cut is dependent on various factors, one of which is blade tip speed (measured in RPM's). The faster the blades spin the more likely you are to get an even cut. Also, tractors that are air cooled depend entirely on the cooling fan to keep the heat down. By running the engine at less than full throttle you are making the engine run hotter.

For a hydrostatic transmission, less than full speed may result in damage because the pump pressure/volume isn't high enough to operate properly.

The difference between full and idle isn't as much as many people think.

"...may result in damage because the pump pressure/volume isn't high enough to operate properly..."

not quite.... reason is that the cooling fan for the tranny is not moving sufficient air to cool it. The hydraulics don't really care about speed - they care about temperature though.... as long as you're not plowing a field - which you're not, because we're talking about mowing your lawn - meaning that your tranny is only propelling itself, you, and your can of beer (not too much of a work-load)

Reason you want full RPM is simple. In order to get a good cut, you need blade tip speed. Slow speed will just bend the grass down... high speed will cut it.... your mowing deck is designed to have a certain speed for mowing.... that speed is attained at full engine RPM. If you don't give it that speed, your deck is operating outside its' design parameters, meaning 'all bets are off' with regards to cut quality, mulching performance, bagger performance (if you have one), etc.

The guys are right about blade tip speed but there is also another factor involved. Modern decks have blades that act as suction fans. This suction pulls air in from below. This vacuuming action causes the blades of grass to stand erect so that they can be chopped off evenly. Dropping the throttle lower reduces the amount of suction and the cut suffers.

I've never heard of anyone overheating an engine or transaxle by not running at full rpm. The heat produced by both is proportional to the speed. Lower rpms, lower heat produced and lower cooling required. I could idle all day long and not damage a thing.

However, cut quality and transaxle speed and torque will suffer at lower rpms.

I agree with you machiem.
The main reason for full throttle when mowing is to lift and cut the grass.
I don't own a hydrostatic tramsmission so I cannot comment on the cooling requirements needed.
Honda ,a few years ago , recommended letting their engines idle for five minutes after hard use before shutting down.
This is a practice that I still follow today.
I get tired of hearing that if the motor is not at full throttle it will overheat.

I agree, my homemade plow tractor sometimes works for hours on end pushing dirt and rocks at half throttle or below, using the granny low gear, and i can honestly say, the engine isnt as hot when you shut it off, as my Snapper engine is after mowing at almost full throttle, and they both have the same briggs 11 hp IC engine. Plus the plow tractor engine is also running a chevy car altanator the whole time via another belt.

Belt drive does not imply it will slip.

I have used many tractors with belt clutches and belt-drive hydros where slippage only occurs if the belts are worn out.

Remember, a shaft drive machine has a clutch that can wear out and slip too.

IMHO the biggest problem is usually traction, not drive slippage, though I have little experience with vari-drive systems where perhaps slippage is more of a factor.

Yeah, what wheelhorse said, I think? What question were you answering?

wheelhorse............stop smoking what your cutting

Kawasaki Tech Support told me it is better to run my Kawasaki 18hp AC Twin at High Idle (3400rpm) because the fuel burns cleaner and there is less carbon deposits on the valves and heads. He said any additional wear running it fast over mid range is very little, but the probability of valve problems is greater at the lower speeds. Certainly, while cutting grass, it should always be run at high idle to cut correctly as stated by others. Charles Ranheim

I contacted Kohler about full throttle operation last year. The engineer that responded said that my engine would run cooler at full throttle and he highly recommended it. However, I agree that blade tip speed and suction are probably more important factors when mowing grass. IMO, mowing grass should always be done at full throttle.

Owner's manuals normally recommend operating at full throttle to ensure adequate engine and hydro cooling. You can believe the engineers or not...the choice is yours.

-Deerslayer

Just as a side point, when we say full throttle we mean full governed throttle. It is not like the engine is running wide open, it is running at 3500RPM give or take. It is not like constantly running a car at red line.

I appreciate all of the comments. My JD GT 225 is near the bottom of the GT series. All and all its been a pretty good machine for me. I don't work it too hard, an occasional pulling of a trailer with some topsoil, an few aerations, and lawn rolling and of course the weekly lawn care. Thanks again.

Mostly good info with a little misinformation. Engine should be run at full governed speed. Shift to lower gears or use the hydro pedal to control ground speed.

Engine will overheat if operated at load at lower speeds. Although heat build up is proportional to speed and horsepower, cooling air is less. Every air cooled engine I know uses a centrifugal fan on the flywheel. Airflow increases at the square of the engine speed. Hence if an engine fan delivers a given amount of air at 3600 rpm, airflow is 1/4 at 1800 rpm.

Water cooled engines do not apply.

This is rubbish! The heat increases to the square of the increase in engine speed, or more. Many of use in the commercial mowing business set the governed engine speed to 3200 to 3400 RPMs so that the engine will last much longer and use less fuel. Have you ever seen an air cooled engine over heat, if it has been kept clean?

A lot of good theories about heat and squares of speed etc....

The rule for fans is that their air movement is generally a linear function of speed (except at the ends of the design range: really low speed, and really high speed). Namely, you move 2x the air at 2000 RPM, compared to 1000 rpm. Has to do with blade speed (tangential speed at the tip) etc... long explanation.... trust me....

The real rule for heat dissipated in an engine is:

Heat dissipated is equal to the specific efficiency at the given RPM (ranges from 0% - at idle, to approx. 50% - the best an internal combustion gas engine can do), factored by the actual load on the engine at the time.

Because these engines are designed to be constant speed engines - by means of being governed at a standard operating RPM such as 3400 or 3600, they are designed to have the highest efficiency at that point. But efficiency is a percentage, so if the efficiency is 50%, and the load happens to be 1hp (746 watts), then approximately 746 watts of heat are dissipated (1492 watts of power come from gas, half goes to do work, and half goes to heat - thus 50%). If at the same time you are doing 10HP of work, then you are dissipating 7460 watts of heat. Note that the cooling fan is spinning at the same speed (say 3400), so the forced air has to carry away 10x the heat.

Now, if you are doing the same 10hp of work at say 2000 RPM, and lets assume the efficiency at that point is 35%, then out of the total consumed gasoline output of 21314 watts, 7460 watts goes to do work, and 13854 gets dissipated as heat. Now, your fan is moving at 68% of the air that it does at 3400, and yet it has to carry away 13.9KW of heat, instead of 7.46KW for the same 10HP of work at 3400 RPM. Almost double the heat!

Engine engineers forsee these instances, and they also account for dirt on cooling surfaces (reducing heat rejection efficiency) etc.... So that the fans are designed to move way more air than necessary.
What that means is that if you're mowing grass with your 20HP tractor, consuming maybe 10HP in reality (because that's all it takes to cut grass), then you are safe. If you're pulling a trailer full of fire-wood, where only 2-3 HP goes to the wheels (yes that's all it takes... I can show you the torque formulas but that would make this message even longer), you are WAAAAAY safe - that's why you can run low RPM when "not mowing", and nothing gets very hot. In fact, in many instances you can run the engine at idle, and pull that trailer - albeit not too quickly because of the gears.

While mowing, you want speed for mowing blades not the engine.

I repeat, have you ever seen an air cooled engine overheat, that was clean and not overloaded? Also 1 hp is a theoretical 746 watts, (I am a physics)and I don't think anyone has ever achieved that. A practical hp is 1,000 to 1,200 watts.

At a reasonable blade tip speed, and we may argue about what that is, the ground speed of the machine has a greater effect on the quality of the cut than ANYTHING else.

In 30 years I have never seen an air cooled engine damaged from overheating that the sheet metal was not FULL of grass. That has more effect than anything else in my judgement.

"Also 1 hp is a theoretical 746 watts, (I am a physics)and I don't think anyone has ever achieved that."

Except Sears:-)

I've seen one overheated due to low speed. All the plastic parts inside (gov gear, dipstick) melted. The cooling fins were perfectly clean.

I would also beg to differ on what commercial cutters do. They normally bump their RPM to the most they can get (3,600 - 3,700 rpm) to get the most power, blade speed, and ground speed they can.

"...1 hp is a theoretical 746 watts, (I am a physics)and I don't think anyone has ever achieved that. A practical hp is 1,000 to 1,200 watts...."

It's a good thing James Watt was an engineer, and not "a physics" (physicist to be correct).... 1 HP is 746 watts. PERIOD. I'm presuming when you refer to 1000 or 1200w, that you mean the power that 1 horse is able to produce?... if yes - then I'd challenge that and bet a horse can produce more like 5-10hp. If you are not referring to horses, then you are flat out wrong. 1hp is 746W - and that's as practical (mathematical and physical) as it gets.... there is no other unit of HP. Thinking that there is a "different unit of HP" would be equivalent to saying that 1 "practical" yard is actually 3.75 feet. That's just stupid.

Wally,

I think what he is saying is that to produce 1 HP OUTPUT real engines require more than 1 HP input since no engine is 100% efficient.

If I am correct Bill is saying that for 746 watts out the typical power consumptions is about 1100 watts.

746/1100 = 68% efficiency which is pretty good.

Obviously INPUT wattage is relevant to heat generation unless one does a separate efficiency conversion in the formula(s).

And it has been reported , "running the engine at closer to goverened speed helps to maintain the battery within a usable state of charge".I very seldom, in about 50 years, find any need to run any engine at WOT.Exception.In HOT dry weather a "loaded" B-52 bomber requires WOT plus water injection, and a L O N g runway, to obtain calculated rotation / lift off ground speeds. But that is another story.tbk

I will keep that in mind in case I ever use a B-52 to mow my lawn.

"Have you ever seen an air cooled engine over heat, if it has been kept clean?"

As a point of interest, exactly what is meant by overheating? I don't recall ever seeing an air-cooled engine with a temp gauge. Overheating could cause engine failure as described by engine tech (melted parts). But couldn't cronic overheating cause longer-term 'partial' failures over time; i.e. valve problems, blown head gaskets, etc. These failures may never be blamed on 'engine overheating' per se, but other causes such as hours, studs vs. bolts, wrong oil, cheap gas, Green vs. Red, Briggs vs. Kawa, etc.

Wally2q, you're a voice of reason and practical experience on this forum, what say you?

Wally2q: You are correct about how I incorrectly spelled physi·cist. "(f¹z"¹-s¹st) n. Abbr. phys. A scientist who specializes in physics." It has been a long time but my old test still defines hp as the amount of power required to lift 33,000 pounds 1 foot in one minute or 550 ft pound in one second. James Watts used that term so that people would have some knowledge of the amount of power he was speaking of. Also it was generally agreed that an average horse could develop about 0.7 hp for some length of time. Don't forget that hp has TIME as a function of it.

It is true that 746 watts is an electrical equivalent of one hp, however, look up the power consumed by a good one hp electrical motor. I find a Dayton 1 hp tool motor consumes 13.6 amps at 120 volts or 1632 watts. That is the reason I, and others, call it a theoretical hp. In the generation of AC power 746 watts is used as a standard of one hp. It works ok there, but not when you try to get that hp back out of it.

The reason some vacuum cleaners can get away with advertising a 3 1/2 hp motor running off 120 volts is the crazy way they measure it. They measure the energy stored in the impeller, in this case really a flywheel, for only a short fraction of a second. Forgive me, it looks like I am going to have to use my spell checker more frequently!

"I think what he is saying is that to produce 1 HP OUTPUT real engines require more than 1 HP input since no engine is 100% efficient."

yes - that's I said what I said in my earlier post:
"....if the efficiency is 50%, and the load happens to be 1hp (746 watts), then approximately 746 watts of heat are dissipated (1492 watts of power come from gas, half goes to do work, and half goes to heat - thus 50%). If at the same time you are doing 10HP of work, then you are dissipating 7460 watts of heat...."

In fact, when I dug into it some more, gas engines are even less efficient - less than the 50% that I suggested... see link:
http://en.wikipedia.org/wiki/Internal_combustion_engine#Engine_Efficiency

Bill in NC you are correct about the origin of the HP.... here is some more on the story:

Mechanical horsepower

The term horsepower was invented by the engineer James Watt in 1782. Watt (1736 to 1819) is most famous for his work on improving the performance of steam engines.

Watt was working with ponies lifting coal at a coal mine, and he wanted to define the power available from one of these animals. He found that, on average, a mine pony could do 22,000 foot-pounds (lift a bucket of coal weighing 22,000 lb. a distance of 1-foot) of work in a minute. He then increased that number by 50 percent and fixed the measurement of horsepower at 33,000-foot-pounds of work in one minute.

Under this system, one horsepower is defined as:

1 hp = 33,000 ft·pound-force·min−1 = exactly 745.69987158227022 W

But as you see above, 1 HP still equals 746 watts... electrical, mechanical etc....

As for your comment regarding electric motors - you're missing 1 important factor... it's called "Power Factor" which defines the vector sum of "reactive power = Vars" and "real power = Watts" - one being 90 degrees out of phase with respect to the other - of course talking about Alternating Current. Vars contribute to current, but do zero work. Watts (or HP) is "real power" only, but the current pulled by the motor is the vector sum of the two, so 13.6A you are referring to does not do equal to heat plus mechanical work. In fact, a typical electric motor is 80-95% efficient, so a 1HP motor will actually consume 785 - 930 watts, while drawing 13.6A Which results in a line draw of 1632 VA's... the unit VA simply translates to Volt-Amps - you'll note that the unit is not "watts"....

As for Vacuum cleaners, what you see is "peak power".... a typical 15A circuit can provide povide approximately 2HP. However, you can overload a fuse or circuit breaker for several seconds, or even several minutes (depending on the overload level). These motors sometimes when loaded, will draw more current than the nominal-continuous rating - when the mechanical demand is present. All the 3 1/2 hp rating means is that the motor can momentarily produce that level of output...... the explanation is lenghthy, but in short - the motor magnetic circuit "capacity" controls maximum torque... if the torque demand is present, the magnetic circuit will either overcome the load and maintain shaft speed, or not - resulting in the motor slowing down. By stating that the motor is 3 1/2hp rated, means that it can produce more torque AND sustain speed, than a say 2HP motor - but again only when the demand is there.... it does not mean that the motor is producing 3 1/2 HP the whole time.... just like the 350 in your shiny new vette does not actully produce 400hp when you're cruising the boulevard... only when you punch it!

Oh yea - don't worry about spell checkers... I was just ribbing you!

Air cooled engines generally don't seize like a liquid cooled equivalent when overheated. Reason is that air cooled engines have a broader operating temperature range, so therefore are designed with greater tolerances to allow for thermal expansion of parts. You can overheat the engine though... just remove the fan off your tractor and run the engine for a while... see what happens.

Things is that generaly that does not happen... however, when you get beyond the design temperature of the engine, you will start seeing effects such as coking of oil in various parts such as rings etc... you see oil viscosity change due to temp., to a level outside of design limits, which can increase rates of wear of various parts... there are a lot of hidden effects that may not manifest themseves for a long period of time.... however - engine engineers do push the design limits quite far, because they know that there are people like me out there - people that abuse their machines - they just put the gas in, and go... oil level too low?... oh well.... cooling fins blocked?... how would I know?... just turn the key and start mowing - LOL... it's true though....

P.S.: I actually take care of my stuff....

IMO, it's inconsistent to store a tractor in a garage or shed, religiously change its oil and filters, and keep it clean and in overall good condition then potentially abuse it by low throttle operation. On the other hand, if you abuse your tractor in other areas, running it at low throttle isn't going to make much of a difference. Like I said, that's just my opinion.

Different strokes for different folks...

-Deerslayer

If it takes 1HP to overcome friction and keep the machine clicking away, while your tractor is spinning at idle, parked, and with no load.... then arguably it is dissipating maybe 1000 watts doing so... given even the small amount of air movement, it probably would not rise to a temp., high enough to cause problems.... meaning you could idle it all day long.... just like someone else already mentioned it earlier in a message....

Read your owner's manual and do as it says and you should be alright.

Wally, I am aware of the power factor calculation. It is often defined as:
(pf = watts average power/rms volts*rms amperes) and is most often very near 1 on single phase AC and is 1 on DC, so I frequently ignore it. Possibly should not, but it does not change my general calculations more than about 2%. After punching 746 watts into Google, I came up with something I don't recall seeing before.
"Curiously, a horsepower is defined as 550 foot-pounds per second, which is about 746 watts. The idea of "horsepower" has a more glamorous sound to it, though, and it's possible to say things like "four horsepower electric kettle" instead of '3 kilowatt electric kettle'.

The reason why the horsepower got defined as about three-quarters of a kilowatt is because it's based on how much work you can get a horse to do in a day. The definition of horsepower dates back to the the time when pumping of water out of mine shafts could be done by having a horse on a treadmill or by the more modern method of using an engine. Industrialists considering whether they should upgrade to steam power could be tempted by how many horses could be replaced by a steam engine and therefore how big a cost saving this represented. So, where a place previously required a team of eight horses to be kept, looked after, and be put to work in turns on different shifts, the size of steam engine required to pension them off would be eight horsepower." But I still do not know where the damn 746 watts came from. Now in this definition there is the factor of work over some period of time. The only place I have found it useful is in expressing the output of a generator in hp, and even then you divide the output wattage by this mysterious 746 watts. I am about to wonder if it does not relate back to steam power in some way. Enlighten me, please. Keep it simple, I am a simple person. Bill

I run mine at WOT all the time while cutting grass. I also have a tachometer on mine and it shows 3250 rpm with the pto engaged give or take 50 rpm. This is on a briggs vanguard 14 hp twin. The only time i dont run it at full throttle is while pulling a cart or something like that. Then, I just run it at about 2500 rpm or so.

Actually, my previous post explained it... perhaps the lenght of my message caused it to be glanced over... I know - I am guilty of it too....

PF being near 1 for 1-phase motors is not nearly as true as some sources would lead you to believe... in fact 0.7 to 0.9 is typical. Theoretically, it can never be 1.

As for Watt and the definition of 1HP, it's basic arithmetic.

force: F=ma
work (or energy): W=Fd
power: P=W/t

where
m = mass
a = earths' gravitational acceleration (a constant)
d = distance
t = time

therefore: P=mad/t
expressed in the trusty metric units....

so,

33,000 lbs = 14982 kg
1 foot = .3048 m
a = 9.81 m/s/s

P= 14982*9.81*.3048/60 = 746 W

that's where 746 comes from.... whether is mechanical or electrical does not matter... work is work, power is power. Watts being the unit for power does not mean electrical power. It can be mechanical power too. It's just a unit of energy expended per unit time. Energy can be thermal or mechanical as well... just the fact that the we use the formula W=f*d which is Energy (or Work) equals to force * distance (nothing electrical about it), and Force = mass * gravitational acceleration (again, nothing electrical about it).

And if you would like to understand the link between an electric heater and a mechanical machine, then imagine that all those little electrons that are trying to flow down that heater-element "pipeline" encounter so much friction, that they produce heat.... the friction causes a "backup" or traffic jam of electrons that goes all the way back to Niagara Falls generators - where those electrons are being pushed by the magnetic field in the generator, but they can't go anywhere because of the traffic jam..... so the magnetic field is pushing, and for every action there is a reaction, and so the field pushes back on the rotor part of the generator, which then transmits that push-back into the water wheel or impeller, which then pushes back onto the water column, and all the way back up the river.... The generator is where mechanical "power" is converted to electrical "power"... all equally represented by the unit Watt or HP.

You already know that power also is defined in electrical terms as P=I^2R, where I = current, and R is the resistance to electron flow.... manipulating the formulas you can create a relationship between voltage & current, and force & distance etc....

If it's considered "abuse", why do the manufacturers give you the ability to change RPM in the first place? I think they'd at least make it more difficult than leaning forward in your tractor seat and moving a lever up or down. Perhaps they let you change RPM so you damage your engine and transaxle and then have to buy more of their stuff?

I've never heard of anyone overheating an engine or transaxle by not running at full rpm. The heat produced by both is proportional to the speed. Lower rpms, lower heat produced and lower cooling required. I could idle all day long and not damage a thing

Anyone who has ever owned an air cooled car (especially one wiith a cylinder head temperature gauge) knows what you say isn't true

Running lower RPM's under load can overheat the engine and the 'solution' is more airflow over the cylinder head . . . . done by increasing the engine RPM

An aircooled car with plenty of power can climb into the mountains in top gear at 70 MPH with no problem . . . . other than overheating. Running 70 MPH in third gear instead of fourth will cool the cylinder heads by 150 degrees or more, as measured on the head temp gauge

Don

"If it's considered "abuse", why do the manufacturers give you the ability to change RPM in the first place?"

It could be the next 'RIO'- type device that manufacturers install on lawn equipment....idle or WOT selector control with no variable setting. And then there will be endless discussions on this forum about the 'best' way to over-ride or disable the feature. ;~)

t

Engine overheating could be indicated by a white blistered spark plug, among other things.

"If it's considered "abuse", why do the manufacturers give you the ability to change RPM in the first place?"

My Kohler Magnum 12 manual indicates to start at half throttle, run at full throttle, and shutdown at idle.

-Deerslayer

Wally: I appreciate your explanation of the 746 watts equalling 1 hp. I checked all 6 of my physics text books and found nothing addressing this. Only 550ft lbs/sec equaling 1 hp. Although I was well aware that 746 watts was considered 1 hp in generating. Now that I think about, I have seen some English of German engines expressed in KW output.

I too am a EE, however, I had almost no education in power generating or distribution. My speciality and education was in micro electronics, thus I am quiet familiar with Ohm's Law.
Now an interesting question: How did James Watts make his early steam engine run when he had almost no steam pressure at the time, too little to push the piston down. This was because he had no pressure vessel that would hold steam pressure of any appreciable magnitude. I ran across the answer to this in the Henry Ford museum.

Engine overheating could be indicated by a white blistered spark plug, among other things.

Hey Bill.... perhaps this thread should be renamed....

in any case, Watt didn't like high pressure steam because it was dangerous... low pressure steam engines used the condensing principle of cooling the steam (by means of water injection) forcing it to condense and create a vacuum - which would then "pull" the piston in the cylinder. In fact this concept was not invented by Watt. He only improved it, by using an external condenser - rather than using the cylinder itself.... this external condenser improved that particular steam-engine configuration efficiency significantly....

cheers!

Many years ago I completed an apprenticeship as a Marine Boilermaker.
I remember in our "main" textbook (Introduction to Marine Engineering) that something like 70% of the power produced by the ships steam turbines was produced at a "pressure less than atmospheric".
This was a WWII era book, where most "modern warship" boilers were nominally 600 PSI. Auxillery type ships typically ran lower pressures. Of course, some didn't have boilers at all.

How do I measure the rpm on a twin without a tach? I have an old hand held Briggs tachometer with a wire that sticks out and vibrates, but I think it was made for single cylinder engines...