Small Miracle: Diesel Fuel Tachometer

MAX Update No. 49: This Diesel Tachometer Is a Small Miracle

6/9/2010 4:48:20 PM

By Jack McCornack

Tags: MAX, 100 mpg, rally, gas mileage, tachometer, tach

I was talking with Bill Bushholz of the Maine Automotive X Team about the upcoming Rally Green ...

Rally Green is a long distance fuel economy rally/demonstration for high-efficiency vehicles, which will take us participants (why yes, of course MAX will be there) from Iowa to California, through amber waves of grain and purple mountains majesty, on public roads and in plain sight to anybody who wants to watch (in marked contrast to the approach of the Progressive Insurance Automotive X Prize competition). I'm all excited, and so is Bill, who is organizing the event.

So after the Rally Green part of the conversation, we were sharing some tech talk and Bill asked if I had a tachometer. A tach is a useful item when you're trying to coax more miles out of fewer gallons, but they're not easy to come by for us diesel drivers. If he'd asked me a year ago, I'd have told him how I made an airplane optical propellor tach work by putting white and black duct tape on the crankshaft pulley, to simulate the flicker of propeller blades. It got me by, but it was finicky and expensive and I had to push and hold a button every time I wanted an RPM number, but now ...

"You bet," I said, "It's what I drive by. It's digital, so it's easy to pick an RPM to cruise at."

"Digital?" said Bill.

"Yeah, a Tiny Tach. They make them for diesel now."

"Huh," said Bill. "How do they work?"

That's an interesting question. Luckily I've got connections. I've been a big fan of Tiny Tachs since they first came out, and thousands of Tiny Tachs have been mounted on aircraft I've designed, so I called Steve Myers, president of Design Technology (the company that makes the Tiny Tach) and asked him.

Tiny Tachs have a quartz crystal clock built in, the gasoline engine version counts the ignition firings against the clock and calculates how many times it would fire in a minute, and that number times two (if you have a four-stroke engine) is your revolutions per minute, abbreviated RPM. The gasoline engine Tiny Tach gets its information from an antenna wrapped around a spark plug wire, so it's totally non-invasive.

But how do they do it with diesel? Diesels don't have any spark plugs, and thus no spark plug wires.

Here's how, and I'm not making this up: the diesel Tiny Tach has a piezoelectric crystal that clamps on to one of the steel fuel lines between the fuel pump and the injector nozzles, and it detects the swelling of the line as diesel fuel (or veggie oil) is squirted through it. Counting the squirts on a diesel is the same as counting the sparks on a gasoline engine.

This is an example of Clarke's Third Law: Any sufficiently advanced technology is indistinguishable from magic. I'd have ordered a Tiny Tach the day I started the MAX project, but since skrying knowledge by squeezing crystals doesn't fit easily into my world view, it never occurred to me they could make them work on diesels. Well, they did, and for less than $100, plus it retains a cool feature of the original Tiny Tach — when the engine isn't running, it displays the total hours the engine has run — and as you see, I've driven for almost a day since I put mine on MAX.

Photo by Jack McCornack

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