Thursday, March 1, 2012

A Girl's Guide to Diesel Engines: The Transmission

I get to scrap the rust off of that. Yay!
First off, I would like to apologize for taking an extended vacation from my little column. There just wasn't much engine work going on, and then I went to Arkansas, far from the Yanmar and my diesel guru. But never fear. I'm back doing glamorous jobs like scraping the rust off the transmission. I've got a wire brush and everything! Tre chic.

But why do I care about rust on the transmission? What essential function does it perform? (Once again, it's shocking that I don't know this stuff!)

Not surprisingly, it's both remarkably simple and potentially very complex once you delve into concepts like torque, which I most definitely will not. The transmission sends the energy that the engine produces to whatever device does the actual work such as the wheels on a car or, in our case, the propellor. Essentially, it's just a metal box full of gears that turn with the crankshaft. When the engine creates power, as I described here, it turns the crankshaft, which has a gear on the end of it that connects to a gear on the transmission ** (See note). These gears turn the gears inside the transmission, which turn the propeller shaft, which then turns the propeller. I know this sounds like the inside of a Swiss watch, so please check out this excellent diagram of what a simple transmission looks like. I found it incredibly helpful. Here's a more complicated diagram from my new favorite book, "The Care and Repair of Small Marine Diesels:"

Perhaps it's not as simple as I thought.

This leads me to my next question though. Why bother with a transmission at all? Why not just have the engine turn the propeller shaft directly? If this were the case, each time you fired up the old diesel you'd find yourself ramming into the dock. You couldn't change your speed, and there would be no neutral. Plus, you'd either have to have a second engine to go in reverse or hop down in the water and change the direction of the propeller. The transmission is a much nicer option. 
Bettie has a very basic transmission. We only have one speed going forward and one in reverse. To speed up, we make the engine turn faster. In a car, however, you might want to go more than 5 miles per hour, so the multiple gears in a car's transmission allow you to increase your speed without making the engine run at such high RPMs that it would rather hang out with someone nicer. Like the guys at the scrap metal yard.

**Note: Vlad keeps telling me that transmissions vary, so please keep in mind that I'm speaking in very general terms and that I'm certainly not an expert on different types of transmissions. That's speaking the obvious, I know.





7 comments:

  1. I just love your girl's guides SO much. I learned more about transmissions than I wanted to while we had ours apart but Mark skipped over the "obvious" parts. Thanks for illuminating me on the "obvious" parts I was ignorant on. I appreciate you not making me feel silly for not knowing simple stuff and taking the time to explain everything from step one!

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  2. My pleasure, Cidnie! I knew nothing about the obvious parts and still know nothing about the more complicated stuff. I've always been interested in engines and am glad I'm taking the time to learn a bit more about them. I think we're going to have to adjust the valves soon, which should be a more complicated post. Coming soon!

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  3. In car repair manuals, the section on transmissions tells you how to change the fluid and then basically says 'if that doesn't fix it, tough luck.' They can get pretty complicated.

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  4. So now I understand why they call it a 'transmission'. It 'transmits' the energy from the engine to the crankshaft. Well, I'm having a severe 'duh' moment here! Thanks for an explanation in plain English. Um, that's your new favorite book? And you can stay awake to read it? I am duly impressed!

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  5. Great post Bettie! I am training a new coworker (we service diesel engines) and he said that women are MUCH better at explaining things because they start with the basics. You don't begin explaining how a transmission works by starting with gear ratios and hydrostatics... keep it simple!
    Anyways keep up the posting they're both fun and educational ;)

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    1. Glad you liked it, Danielle. Like I said, I know very little about engines, but I've been trying to get a bit more educated on the subject. Sounds like you work in the field. Very cool! I should probably be calling you for advice.

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