MAX Update No. 62: Throttle Management

I got an e-mail from a guy building his own MAX, asking about how I hooked up the throttle — which turned out not to be quite as trivial as I thought it would be back in Twenty Ought Seven, when this project began.

The problem is, diesels don't usually have throttles, and this Kubota is no exception. Oh sure, we call it a throttle, but it's not. We tend to call anything that controls power a “throttle” — my gosh, airline pilots call their jet engine power controls “throttles” and boy is that a misnomer on a jet. Throttles are usually found on spark ignition engines, such as the gasoline engines that motivate our automobiles, but throttles are not a terribly efficient way to regulate engine power output, and one of the reasons that diesel engines are more efficient than gas engines is diesels don't have throttles.

Maybe Webster's can give us a clue here. Ah yes...

Throttle \Throt"tle\, v. i.

--1. To have the throat obstructed so as to be in danger of suffocation; to choke; to suffocate.

--2. To breathe hard, as when nearly suffocated. 

We've forgotten what the word means, over the last hundred years. We talk about giving a car more throttle to make it go faster, when we mash our foot to the floor we call that ”full throttle,” but the truth is, throttling is a way to slow the engine down. If you had a spirited draft horse that pulled so hard that you had trouble jogging along behind the plow, you could make that horse ease up by partially strangling it. Well, that's how your car engine works. When you let off the gas pedal completely, the engine is so throttled that all it can do is idle, until you un-throttle it by stepping on the gas.

MU62aTypically, the throttle is a disk inside the air intake with a round shaft bisecting the disk (it's called a “butterfly” because it looks like one, presuming you have a rich imagination — the shaft is the body and the two halves of the disk are the wings). The shaft sticks through the wall of the air intake tube, so the butterfly can be turned from the outside — turn it one way and the disk blocks the airflow, turn it 90 degrees and the disk is edge-on to the air and doesn't block airflow much at all. In between the two extremes, you have varying levels of strangulation, and no, it isn't very efficient, and at low throttle settings, the engine spends more energy gasping for air than it gives out through the crankshaft. That's the main reason we get worse mileage in city driving than we do on the highway...but back to the subject, how do we hook an engine throttle to a gas pedal?

The standard practice for cars nowadays uses an inner cable in an outer sleeve. Pull the cable out an inch from one end of the sleeve, the cable goes in an inch at the other end of the sleeve, no matter how circuitous the path of the sleeve. One end of the cable connects to the throttle pedal and the other end wraps partway around a “cable cam”, which is a half round metal stamping with a groove on the edge to guide the cable.. If you don't know what I'm talking about, look under the hood of your daily driver; I'm pretty sure you'll find a cable cam in there. You push the throttle pedal, it pulls the inner cable, the other end of the cable is attached to the outside edge of cable cam, and the center of the cable cam is attached to the butterfly shaft. The pedal assembly and the cam are designed so when the driver pushes the pedal flat against the floorboard, the butterfly turns ninety degrees, and air can flow into the engine unobstructed.

Like most diesel engines, the Kubota D1105T that powers MAX doesn't have a throttle. Instead, it controls power by limiting the stroke of the fuel pumps (there's a tiny little fuel pump for each cylinder); less fuel = less power.

Since this “throttle” controls a small amount of fuel instead of a large amount of air, it doesn't have to move very much.

MU62bIn fact at full travel, that fuel pump controller only turns about 30 degrees. Compare this with the 90 degrees of a butterfly throttle and you see the problem: if we use the throttle pedal, cable, and cable cam that came out of the donor car (an ancient Toyota Corolla, in MAX's case) the pedal will only move 1/3 as far and be three times more sensitive than it ought to be.

My solution was to cut a quarter circle's worth of cable-attachment-and-guide off the Toyota's cable cam, weld it to a strip of steel, and bolt the assembly to the “throttle” lever on the engine. Of course I had to figure out how long to make that strip and where to put the bolt holes, but fortunately, I remember everything I learned in 7th Grade Geometry (wow, am I dating myself, or what?) and it was child's play (hey, I was 11 back then) to calculate that I needed four inches from pivot point to cable to get the Toyota's throttle pedal in synch with the Kubota's fuel pump lever.

I could have found the same answer through trial and error, but it would have taken me a lot longer...unless you count the time I spent in geometry class.

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