Posted by docmirror on November 30, 2018 at 09:20:40 from (107.213.165.255):
In Reply to: Thirsty tractors posted by larry@stinescorner on November 30, 2018 at 05:35:38:
Who wants to do math? No-one? OK, here's some oats for consumption;
The Otto cycle spark engine(4 stroke gas) runs about .4 Lb/hour/HP of fuel in good repair and well tuned for stoichiometry(air/fuel ratio). Many engines can run at much higher ratios than that and still operate and not foul the plugs too bad. The .4 Lb/hour/HP works out to a air/fuel ratio of basically 15:1. (a bit more, a bit less). If we work backwards from HP, it's easy to see that there is a lot of variation, but take the type of work done, and it starts to make sense.
The Oliver 1800 mentioned above was running ~7GPH under heavy load. The engine was rated 74HP at the PTO. From this we can derive that the pumping losses, and the ancillary losses, and driving the hyd pump, and the trans/PTO gears all takes up about 25% of the power output. Working backward, the Oliver 1800 engine shaft HP was pretty close to 100.
Take 100HP/hour, and divide by the fuel used of 7GPH, and we come up with a air/fuel ratio of: 100/7 = 14.3:1. Running just a bit rich, this engine could be tuned a bit and produce the 6.5GPH number with a slightly leaner mixture. 100/6.5 = 15.4, which is right in the ballpark for our proper stoich mixture.
Any tractor can be evaluated the same. Find the PTO HP, multiply that by 1.25 for the shaft HP, then divide the fuel used per hour, and find the stoich ratio. Something close to 15:1 will be good. Below 14:1 is getting pretty rich mixture. Getting down to 12:1, and now one has a thirsty tractor.
What a lot of folks don't take into account is all the parasitic drag that a tractor runs. Not just the alternator, but also a hefty power steering pump. Trans gears in tractors are big, heavy, solid pieces. PTO drive accounts for a load of HP, and finally the hyd pump is sucking up HP all the time.
Fill the tank full. Run your tractor under load for 1 hour. Stop and refuel, note the exact usage, and check your HP used, and then the air/fuel ratio. If you don't get near ~15:1, change the jets in the carb to a leaner mixture. The plugs should be golden brown, and not black, or dark chocolate.
Diesel engines are about 15-20% more thermodynamically efficient due to less pumping loss, and slightly higher energy density of diesel over gas.
For more reading, research "BSFC", brake specific fuel consumption. Tractors are never going to be 'efficient' just because of the amount of work they do, just sitting still.
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