Horsepower comparison

[Volfandt]

Most all small diesels are indirect injection. I don't know of a direct injected small diesel but that doesn't mean there isn't one out there.
The way it was explained to me was that indirect injection allowed for a better mixing of the fuel/air which allowed for a more complete combustion in the smaller diesels.

The size of the combustion chamber area above the the piston has a direct influence on whether a diesel uses direct or indirect injection.

The smaller diesels having a much smaller combustion chamber area above the piston doesn't allow for an optimum mixing of the fuel/air mixture as well as not maintaining the cylinder temperature at the desired rate. The premixing in an indirect injection engine offsets these shortcomings.

Larger diesels with their larger volumetric capacities have no problem mixing the direct injected fuel with the compressing air and they maintain their combustion temps better to where indirect injection isn't requred.

It all has to do with the size (bore & stroke) of the engine than anything else.

What the happy median is that determines which method is best, is, well beyond me

Volfandt

 

[VistanTN]

I got a little different take ...

I think we all agree that DI sprays fuel directly into the combustion chamber at/near top of compression stroke.

My understanding of IDI is that the fuel is sprayed into a pre-combustion chamber connected by ports to the main combustion chamber, but still at/near top of compression stroke.

IDI was developed to reduce the noise and clatter of DI in smaller engines, but with multi-measured injections and electronic controls, most new diesels are DI (it's a much simpler head casting).

Just my understanding. Mr. Diesel, himself tried injection on the intake stroke and found that timing the ignition point was too variable -- and went to DI at/near top of compression.

VistanTN

 

[neverenough]

I have kind of a basic question regarding horsepower. What is the difference between the "horsepower" in a big lawn and garden tractor and a SCUT and CUT?
In other words how can a CUT with 25 HP do so much more than a garden tractor with a comparable rating. I've been looking at a lot of brands and it seems that 25-30 HP can do a lot, but it still sounds "small" to me.

Also can someone compare/contrast what a "newer" model 50 horse can do compared to an "older" 50 HP tractor (that probably weighs twice as much). At least this I can somewhat relate to in regards to new technologies.

It takes X amount of HP to do Y amount of work. Given that, the hp to, say, run a mower, is equal whether it is with a 3000lb tractor, or a 1000lb tractor. So, in order to run, say a 60 inch deck, it takes, as an example, 15 hp. Now you need hp to run the transmission, to move the tractor. You will also need hp to operate any add on's like a bagger. So, let's use 2 Deeres as examples, to see how this comes together.

Take a 790 CUT with 27 hp and MFWD. The tractor weighs 1930lbs. You can only put a 60 inch MMM on it. So now this 1930lb tractor is out mowing with the 60 inch MMM.

Now, you can take an X728 GT with 27 hp and MFWD which weighs about 1000lbs and put a 62 inch deck on it. PLUS, you can put a power flo bagger on it which runs off the mower deck, and go mowing. The lighter tractor doesn't need the hp to move the lighter chassis, but it needs the hp to run the power flo bagger not available on the bigger tractor. This is why the hp is high in some garden tractors. It's not to move the chassis, it's to operate the attachments available for that model tractor.

There is more to it than that, but, I hope this is a good example of a reason why hp can overlap in different size chassis.

Here, I'll give you another example of hp to wieght differences. In the late 60's early 70's Deere offered an ag tractor numbered 4000. The tractor had more hp than the 4020 yet weighed much less. The reasoning behind the tractor was the motor had less of a tractor to move, so it could pull more at a faster speed (plows and bigger discs) than the 4020.

 

[neverenough]

DI is more expensive to build than IDI, but it is much cleaner. The EPA forced diesels into DI becouse of emissions standards. It's not a whole heck of a lot better for making power, but it is much easier to make a motor run clean with it. 2-stroke motors are using the technology for the same reason. Evinrude E-tec outboards use it, and some Ski-doo snowmobiles are coming out with DI. (Ski-doo bought Johnson/Evinrude for that very reason)

 

[oldgoat2]

My 2 cents....

I have understood torque is the more useful measure an engines ability to do work.

Gas engines generally have torque ratings about equal to their HP rating, give or take 10%.

Diesel engine torque ratings are 2-5 times their HP rating depending on CID, turbo, and intercooling.

Some in this forum have expressed feelings that the HP rating is useful mainly as a marketing tool.

 

[morningwood]

I agree with you that torque is more valueable that HP when it comes to tractor and overall grunt work.

Since I started reading about tractors a few years ago I wondered why they rate them in PTO HP instead of PTO torque. If I remember from College Physics 101 torque is twisting power and since your PTO is turning ( twisting ) why not measure that amount of force ??

Scott

 

[Backup]

horsepower = (torque * RPM) / 5250

Torque will always equal HP (and HP will equal torque) at 5250 RPM. Diesel engines produce more torque at lower RPM versus gas engines, due to the longer stroke (longer crank arms provide more leverage). Gas engines generally produce more HP than diesel because they can spin to higher RPM, so even though they are making less torque the higher RPM creates more HP. Diesel engines usually top out near 4000 RPM or so, due to the characteristics of diesel fuel combustion - it has more energy but combusts more slowly than gasoline, therefore limiting the raw speed of the piston (and hence engine RPM).

Diesel engines are more efficient than gas engines (convert more energy from fuel into mechanical energy) because of the higher compression ratio - nothing else. High performance gas engines usually have higher compression ratios than standard gas engines as a way to convert more fuel energy to mechanical energy but require higher octane fuel to burn properly. Higher octane gasoline resists compression ignition better, allowing the use of higher compression ratios. Diesel fuel has a cetane rating which is the opposite of octane. Higher cetane means the diesel fuel is more able to compression ignite. But a high compression gas engine is around 12:1 where a diesel is around 18:1 (or higher). When a gas engine 'knocks' or 'pings' it is because of the fuel igniting before the peak of the compression stroke. Old diesel engines relied on this technique to run - the fuel mixture (brought in on the intake stroke) would ignite near the top of the compression stroke but could be a little early. That is not the most efficient method nor the best way to make power, so direct injection at the top of the compression stroke is used (and it is much cleaner). Modern diesels generally use a high pressure injection system that is precisely metered, measured, and timed with the compression stroke. Modern diesel injection systems may use a series of small fuel pulses during the compression/power stroke to maximize combustion properties and reduce emissions. Small tractor diesels are not that advanced though.

Because diesels rely on heat from compression to ignite the fuel they generally have more difficulty starting in cold weather. Glow plugs are simply a way to inject some heat into the combustion chamber to aid starting in the very cold. They do not act like spark plugs and actually ignite the fuel though. Modern diesels with direct injection and very high compression can usually start without the use of glow plugs (using winterized diesel fuel) down to about 0 degrees F or so, although glow plugs help. Since diesels are so efficient (again, the compression ratio) they may not produce enough heat when idling in very cold weather to stay at normal operating temps. Glow plugs may cycle on if the computer determines that more heat is required even if the engine has been idling for some time.

Diesel engines operate most of the time in a lean fuel/air state. Gas engines must operate at exact stoichiometric ratios of air/fuel (14.7:1) for maximum efficiency and lowest emissions. Gas engines use throttle plates, air flow sensors, fuel injectors, and a computer to map the exact amount of fuel to use based on operating conditions to keep the fuel/air ratio exactly where it should be. Diesels inject enough just enough fuel to operate regardless of the amount of air or the air/fuel ratio. That's why it is a misnomer to say 'full throttle' on a diesel engine - they don't have throttle plates to restrict air intake (usually). That's also why there is very little intake manifold vaccuum on a diesel. This is fine until the engine is trying to produce maximum power, or injecting as much fuel as possible, because at some point the limiting factor becomes the amount of oxygen available in the air charge to properly burn all the diesel fuel. A diesel will begin to run rich and that's usually when you see it exhaust black smoke or soot (incomplete combustion of fuel). Turbos force more oxygen into the charge and therefore allow more fuel to be injected, making more power. A diesel not under load should not smoke - if it is it either has a restricted intake (clogged filter), improper injection, or is too cold to burn all the fuel.

Back to the original question - a HP is a HP and a ft/lb is a ft/lb. The characteristics of a diesel engine are such that they produce more torque at lower RPMs than a gas engine. Gas engines usually spin to higher RPMs and therefore make more HP than diesel engines ( HP=(torque*RPM)/5250 ). Diesels are well suited for tractors because of the high torque @ low RPM, efficiency, low operating RPM, and generally robust design.

 

[3RRL]

Backup,
That is the best ever explanation of the difference between diesel and gas engines I have ever read, and how they work!
Thank you,

 

[RG_Upton]

Diesel Fuel also contains about 10% more raw energy per volume than Gasoline. This is also a factor in the increased efficiency of Diesel engines over the Gasoline counterparts.

Russ

 

[Backup]

Diesel Fuel also contains about 10% more raw energy per volume than Gasoline. This is also a factor in the increased efficiency of Diesel engines over the Gasoline counterparts.

Russ

True, diesel fuel does have more energy per unit than gasoline. But the efficiency of the engine, that is how much of the potential chemical energy in the fuel is converted to useful mechanical energy, is greater in a diesel due to the higher compression ratio. The higher energy content of the fuel doesn't allow the engine to be more efficient, at least in the strict definition. Now, in the real world, we all consider a more efficient engine to be a more powerful engine, and in that sense the additional energy content of the fuel contributes to the additional power output capability.

I've read that the most efficient gasoline engines convert around 40% of the potential energy in the fuel to mechanical energy. Modern diesel engines are around 50% (or 20% to 30% better), or so I've read.