Miss Daisy report and "What is a gallon of gas worth?"

Driving Miss Daisy went fine.

I was able to negotiate the hazards of East Lansing traffic and I killed some of the time in the bagel shop next to the PT shop. I ate a chocolate-cheesecake muffin and people-watched while waiting.

Miss Daisy is very pleased with the intake session and with her PT person. The PT person was overwhelmingly positive. "Oh! You have excellent muscle tone." and "Very good baseline range-of-motion".

It is awesome when a patient goes into PT and they are not starting from 200 yards behind the starting-line.

Oil

I got to thinking about what five gallons of gasoline or diesel fuel is worth.

Back in the middle-1980s, automotive engines that had BSFC numbers of 0.40 pound of fuel consumption per horsepower-hour were as common as house-flies. Small engines are inherently less efficient because they lose more heat to heads and cylinder walls relative to engines with larger cylinders. The "adiabatic" i.e., no heat loss, expansion of the heated gasses in the cylinder are less-adiabatic.

Since I am too time-stressed to research BSFC numbers for small engines, let's assume that the best you can do is about 0.60 pounds of fuel per horse-power hour.

A gallon of gas weighs about six pounds.  Six divided by 0.60 gives you an upper-limit of 10 horsepower-hours of work. 

A gallon of diesel weights seven pounds and the engines are potentially more efficient because their greater compression ratio can squeeze more "adiabatic expansion" out of each cycle.

This is not a traditional BSFC map. It charts thermodynamic efficiency. Most efficient region is the gray plateau on the upper-right portion of the surface. Idling and partial load are the narrow strips in the lower-center portion.

Matching the load to the engine's output is a major issue. Peak efficiency is only achieved at relatively high loading. That is, near open throttle at medium-to-high RPM. 

A motor powering a generator that has little load will not give you 10 hp-hr per gallon of gas! A motor running an irrigation-pump (continuous duty) that is intelligently sized for the application will come close to giving you 10 hp-hr per gallon. Most other applications will fall somewhere in the middle.

Food/gardening

The decal on the engine that drives my rototiller claims to be a 6.5hp engine.

It rarely takes all 6.5hp to move the tines of the tiller through the soil. Consequently, I am not going to get the work of one-horse cultivating soil for ten hours out of one gallon of gas.

However, I might get the equivalent amount of work of a horse working for three hours cultivating. An added bonus of the tiller is that it takes less space to maneuver it at the ends of the rows. That is a big deal when you have to fence your garden against wildlife.

And that jibes fairly closely to what I see in terms of how often I have to fill the gas tank. I need to refill the gallon tank after every three times I use it for weed-control where it is a quick, shallow pass and the soil is not laced with mature, woody, plant roots.

Five gallons of fuel, if dedicated to running a tiller, would be more than enough to keep a very large garden mostly weed-free for the growing season. How large? 

If two gallons of gas is enough for six passes with the tiller over 6000 square feet of midwestern loam, then five gallons of gas would suffice for 15,000 square-feet or 1/3 of an acre. That should be more than enough to keep a family of six in vegetables even if the rains were not-the-best.

If the rains ARE good, there will also be a surplus that can be shared with family or traded for other items that are needed.