My old Honda CRX Si ran at 3000 rpms at 60 mph on the highway in 5th gear. I drove many highway miles running 3500 rpms to keep up with 75 mph traffic. It was a great car but it needed a 6th gear.
The only gain I can see with the throttle plate opened,is that the engine carries less manifold vacuum and doesnt have to fight the resistance(only reason a gas engine holds back better then a standard diesel) the older cars had guages to remind you(or lights) to keep the manifold vacuum high for max economy.
@ Jes,I got rid of a Ram 50,basically because it didnt have an overdrive gear(4 spd,small tires and low final drive ratio)The 86.5 Nissan seemed much better.
There are other things going on. Get into the throttle and spark can be retarded from 35 down to 5 degrees BTDC. This hurts mpgs.
Glad you mentioned that,wonder how they coped back in the day before automatic advance(arent some aircraft motors,manually controlled?)
^Pretty much anything run off a magneto is limited in spark advance/has no spark advance. Airplanes spend 80% or more of their time at steady cruise RPM and power, so you design for that condition, and resign yourself to sub-optimal operation outside that envelope. An auto is more demanding, in that it operates at a variety of RPMs and power settings.
Some magnetos had impulse couplings that automatically retarded the spark for starting, so that old Farmall wouldn’t break your arm while trying to crank it.
But the main function of an impulse coupling was to hold back the magneto shaft making the magneto drive wind up a spring and when it was time for the spark, the magneto shaft was released and the spring gave the magneto a very fast half turn for a hot spark. This is what is responsible for that cluck-cluck-cluck noise while the engine was being cranked.
When the engine is running, the impulse coupling automatically disengaged and the magneto reverted to direct drive.
I believe that airplane mags also use impulse couplings.
I do know you could “prop” start some planes by yourself,by just charging a cylinder on TDC,when you gave it the spark,it would kick over enough to run(sounded pretty dangerous to me-if someone besides yourself had to turn the prop by hand)pilots had to understand what was going on with their engines for a variety of reasons.
Mazda has done some work on starting engines from compression by stopping them in exactly the right spot. That was a few years ago, and I haven’t read anything about it recently, but it made sense. It was attractive mainly for hybrids where the engines start and stop a lot. They have also done a lot of work on engines that operate as either diesels or with spark ignition. That would make for the best of both types, with the power of a spark ignition engine for acceleration and the efficiency of a diesel most of the time. Various makes have worked on the concept, but Mazda seems closest as they currently make a high compression spark ignition engine that is closely related to their diesel engines. Mazda does an amazing amount of research for one of the world’s smallest automakers (excluding the exotic sports car companies).
I know big diesels often have pneumatic, not electric starters–but couldn’t you start an engine simply by dumping compressed air into whatever cylinder is ATDC on the “power” stroke? You could eliminate the “starter” motor entirely!
Dont see why not,thats how they used to start a 50 HP,Fairbanks -Morse engine at the antique show in Bridgewater,VA.You would need a storage tank and some piping and ,valves,maybe they should pickup on the idea again.(they used to start the turbines on a B-52 bomber with a black powder charge in one of the stages,in the engine)another thing Mean,they could accumulate enough compression of the engine to “charge” the starting system.One of the reasons for pnuematic starters on the big diesels was longetivivty and of course you could “jump” start one with any reasonable compressed air source,good idea.
I believe the Russians had air starters on most of their WWII era prop planes because batteries were unreliable in Siberian winters.
There are two kinds of pneumatic engine starters. One uses a pneumatic version of a starter motor, which works and sounds like a common pneumatic air tool while being used, in place of an electric starter.
The other system distributes compressed air to the engine’s cylinders with an air distributor. The second system is used in huge ship diesels. These engines also can be started in the reverse direction allowing direct drive to the prop shaft.