Dear Ole Mechanic;Hit and Miss Engine

I went to an antique tractor and engine show recently, and I have some questions about some of the engines that I saw.  The engines in question would sit there turning over but would only fire and pop every once in a while, yet they continued to turn.  I heard them referred to as “hit and miss” engines.  Why didn’t they fire every time?  Why would you want an engine to misfire?  Wouldn’t it do more work and run faster if it fired every time?  Also the engines seemed to run a long time on the little glass of fuel on top.  Why do today’s engines use so much more fuel?  Can you explain?

They were before my time

 

Dear Before My Time;

Hit and Miss Engines were before my time also but, yes, I can explain, as they are very near and dear to my heart--as are old tractors.  Please realize that at least one of the engines at the show was built in the 1890’s, some five to ten years before Henry Ford built his Quadra Cycle (automobile) and is now about 110 years old.  Most of the displayed engines were from the teens, 20’s and 30’s of the last century.  The engine builders of that time were just experimenting and learning how to make use of internal combustion principals.  A lot of their knowledge was what had been learned on the earlier steam engines, and the internal combustion engine was just in its infancy--or maybe adolescence.

First, the fuel usage.  The little glass container on top did not have fuel in it-- it contained oil.  The oil was allowed to drip slowly to lubricate the piston.  Those early engines did not have an oil pan or oil pump, so a supply of oil was positioned higher than what it was lubricating, so that the oil could just drip down on what needed lubrication.  Any oil that was not burned was allowed to just drip off--it was a “total loss” lubrication system.  Had you looked closer, you would have noticed either oil lubricators like the one on top, oil cups or grease cups on other critical moving parts, and no way to catch the excess lube from them.  The fuel, usually gasoline or kerosene, was usually stored in a tank built into the engine base or sometimes in a small external fuel tank.  While the fuel consumption was fairly low, the horsepower was much lower.  A 1 1/2 horsepower hit and miss engine weighs in at about 150 to 300 pounds.  Of course, it would produce the 1 horsepower all day long for many years, if you kept it from running out of water, fuel, and oil.

The speed of the old engines was necessarily kept low, as the pistons were usually made of cast iron, and balancing was crude to non-existent.  Consequently, the RPM (Revolutions Per Minute) were kept very low to keep the engine from shaking, bouncing around, and FLYING APART.  A 500 RPM engine was a very fast turning engine and still had to be staked down or have a very heavy, solid base.  In comparison, today’s 10 to 20 horsepower lawn tractor engines weigh in at about 50 to 100 pounds and usually turn up to 3600 RPM.

Controlling the RPM speed on the old engines was critical to keep them from bouncing around, shaking, and flying apart.  It is the speed control used in the old engines that gives them the “Hit and Miss” name.  With no work load on them, they would:  Hit (Pop)--Miss (Puff)--Miss (Puff)--Miss (Puff)--Hit (Pop).  This was caused by the speed-control governor that was usually incorporated into the flywheel.  If the speed was faster than the governor setting, the governor mechanism would hold the exhaust valve open.  With the valve open there would be no compression, and without compression the cylinder would miss (misfire).  Once the RPM speed got slower, the governor setting the mechanism would allow the exhaust valve to open and close normally.  That allowed compression, and the engine would fire (hit) normally.  It would continue to fire until the RPM got faster than the governor setting, at which time the exhaust valve would be held open to prevent firing and keep the engine from running too fast.  If there was a load on the engine, such as when the engine was powering a water pump, saw, feed grinder, or generator, the engine would fire every time as long as the RPM did not get above the governor setting.  Now you might ask why the speed was not controlled by the throttle.  The throttle could not be used because they did NOT have one.  A throttle would just have added complicated machinery that could fail or get out of adjustment--plus they had not really refined the carburetor enough to use a throttle-valve.  This was not something you really wanted in an era when speed was usually controlled by “giddiup” and “whoa.”

One point of interest is that many of the display engines owners at the show have worked on the governors on their engines to slow them down even more.  There are several reasons for this. One is so that there is less wear on these irreplaceable antiques.  Another is so that the actual working parts of the engine can be seen easier.  Perhaps the most important reason is to see who can get their engine to run the slowest.  Don’t ask me why, other than it takes a very talented operator to get these old engines to run really slow.  Thank you for your question because as I said earlier the old engines and tractors are close to my heart.  Why?  Because they are so simple to work on!

Herr Professor Nuzanbolts