Why Diesel Engines?
The market for tractors and commercial lawn equipment has been taking small steps each year to appeal to a customer base that's becoming less farm educated and more residential home owner. This type of customer is often confused by the difference between a 22hp gas engine, and a 22hp diesel. After all, they are both 22 horsepower right? Many people are also turned off by perceptions that are largely based on issues 20 years ago. Modern diesels are not only quiet and efficient, but they out perform their gas counterparts by a large margin. We will be comparing two very popular engines, a 22Hp Kohler Command and a 22Hp diesel built by Kubota Engine Company.
The dictionary definition of horsepower is : "A measurement of the amount of work; one horsepower is the amount of work necessary to lift 33,000 lbs. one foot in one minute". So given this one would be lead to believe that a 22hp gas engine should perform exactly the same as its 22hp diesel counterpart. This in fact is true, and under ideal conditions (not under load) both do create similar amounts of power.
The definition of torque is : Force x Movement = Work. The total amount of work that an engine can do is largely determined by the amount of force that it can exert on its drive shaft over a given range of loads, not just at the peak where horsepower is measured.
Imagine if you have a wrench that's 24in long and you push down with 50lbs, you are applying 100lbs of torque on the nut. If you take that same 50lbs but use a small 8in wrench you can only muster 34lbs of force. When operating an engine in a tractor the length of that wrench is constantly changing. To put it in the most simple of terms... A Diesel has the bigger wrench.
The images above are the torque and horsepower curve from a 22hp Kohler Command engine just like is used in a vast majority of consumer and light commercial products. The Kohler engine reaches its peak 22hp at 3600 RPM. However a quick glance at the torque curve shows that its only able to muster a 32 ft lbs of torque at that speed. When put under a light load the engine is unable to maintain its speed and will progressively slow down and loose power. Its torque will peak at 37 ft/lbs @ 2400 RPM where it generates 17Hp, slipping much lower will case it to stall.
The Kubota diesel engine shows totally different results when put under load. Just like the gas engine, a diesel has less torque when running at full throttle, however the results under load are not nearly as dramatic. At a full 3600 RPM the engine has about 37 ft lbs of torque, a worst case scenario, but still more than the Kohler can muster under any circumstance. When pushed hard and slowed back to 2400 RPM we are still left with 19.5 Hp and 41 ft lbs of torque. Given the torque difference its going to take much more strain to bring the engine to this point and even more to bring it to its knees. To go back to the wrench example, the diesels wrench does not shorten as quickly when you push on it.
Honesty in Specification
Another issue that is largely ignored is the way in which these engines are ratted. These companies realize that consumers are easily swayed by horsepower ratings and so they do their best to always stay ahead of their competition. Kohler bench tests using the "SAE J1340" standard which has no air cleaner, exhaust, charging, cooling or fuel pump. Once the tractor manufacturer adds on the rest of the components a substantial drop in performance occurs that skews our conclusions seven further. By using Kohler's bench test instead of real world performance the tractor company can make it appear as if they have more power than they really do. This is a very common practice and can be easily spotted on spec sheets where "As ratted by the manufacturer" is in fine print. Kubota used the "SAE J1349" standard which includes standard accessories needed for real world use. We can assume that our engine performance data from Kubota is the same as what we would find in a production tractor. You will also find that Kubota's 22hp engine is actually capable of producing 25hp if its RPM's are further increased. While this would provide good numbers on paper, real world performance would suffer as this engine is commonly used for more demanding tasks (Kubota's own BX2230) where high torque is more important than high horsepower.
Diesel fuel has a higher energy density than gasoline. On average, 1 gallon of diesel fuel contains approximately 147,000 BTU of potential, while 1 gallon of gasoline contains 125,000 BTU. This, combined with lower operating RPM's and overall efficiency explains why diesel get better fuel economy than equivalent gasoline engine. If 30 percent of passenger vehicles in the United States were powered by diesel engines, the nation would save 350,000 barrels of oil per day.
Our fine government has decided to tax gasoline heavily., on average you pay 40-50 cents per gallon that you pump. Since diesel fuels are used in off-road applications some distributors will offer "off-road diesel". These type of fuel is the same as what's offered by any gas station, but its dyed red to indicate its intended use. Over the long term its possible to save a considerable sum of money by going this route. There has been a real surge of commercial lawn care companies moving to diesel and they have been reporting a savings as much as 30-40%.
There are a number of reasons why tractor trailer trucks always run diesel engines. Torque for pulling their loads is certianly the first, but in a close second is longevity. Its not uncommon to see highway diesel trucks driving over a million miles before the owner will consider a trade in. The same holds true to diesel tractors. Its safe to assume that a small diesel engine is good for at least 5,000 hours. In car terms that 300,000 miles (@60mph). Consumer gas engines are considered end of life around 1500 hours. Maintenance dictates the true longevity of either model, oil changes are important!
Nothing is perfect! Clearly we have presented a solid argument showing that an investment in a diesel can be very worthwhile. Often times, the purchase is exactly that, an investment. Generally speaking a liquid cooled diesel engine can be sold at about $1000 move that their gas counterpart. Large companies like Kubota have easy access though their own manufacturing facilities and can offer a diesel in their own product at a smaller premium. Small companies can purchase gas engines at a competitive price from Kohler for use in their products, economics of scale help keep the cost down. When you examine diesel offerings you find that there can be a considerably larger price gap due to the extra middleman and low sales volumes. Secondly, environmental emissions are higher from diesel than they are from gas. Modern diesels do a much better job than older ones do and its uncommon to notice any foul order. None the less, the impact on the environment is greater.