I've been meaning to document the vacuum brake circuit for a while now, one of those roundtuit things. Well, someone just asked me to, so here it is.
Vacuum reservoir
The vacuum pump is an old membrane pump that was used on a number of GM cars. Here's an excellent page documenting what models used these pumps. Vacuum pump quide
The vacuum reservoir is an electronics enclosure that I drilled some holes and epoxied some hose fittings into. The lid has a gasket that seems to do a good enough job. The enclosure is about 6"x6"x5" and gives about 3 pumps of the brakes before the vacuum pump comes back on.
The vacuum sensor and switch is a SMC Vacuum switch model ZSE4-T1-25. It is a digital unit with adjustable thresholds and polarity. The output signal is not capable of directly driving a relay so I created a simple circuit to drive a relay, which, turns on the pump.
Driver circuit
At first I tried driving the pump directly with a low side mosfet. I found out the hard way that the pumps negative is shorted to frame so the low side switch didn't work. I added the relay as a simple fix.
My vacuum switch is set to turn off the pump at -65 PSI and to turn it on when pressure goes above -20 PSI. I also put an inrush current limiter in series with the pump as a noise mitigation effort. The pump will make a lot of noise when it first starts up since there is no load on it; the inrush current limiter causes the pump to start up softly. I used a CL-60 inrush current limiter.
There is also a hose on the outlet of the pump, this act as a muffler. I’ve heard you can improve it further by stuffing the hose full of steel wool. I haven’t tried it though.
Another way to reduce noise would be to use a spherical reservoir. The sides and lid of the box I’m using vibrates quite a bit and thus acts as a speaker.
In looking at your reservoir and noting your comments about it, here's an alternate idea for a reservoir.
Go to you local hardware/plumbing store and get a 1' length of 4" dia Schedule 40 PVC pipe and 2 caps. Schedule 40 PVC pipe is rated up to 400 psi. The walls of it are considerably thicker than the electrical box you are using, so the "speaker" effect you mentioned earlier should go away. Also, the PVC pipe is designed for sustained pressure so over time, it will probably stand up better. That's one area you can't afford to have fail.
Whereas your current reservoir is about 180 cubic inches, this new one will be about 150 ci. That can be modified by going to a larger diameter or longer length. For that matter, you could mount a pair of them on the fender well and double the capacity. They could either be hooked up in 'parallel' or 'series'.
Take and mount a single fitting in each cap: 1 to go to the brakes; the other to go to your pump. Glue the caps on each end, being sure to use primer and glue. Now you have what looks like a big "pill" with a fitting on each end.
Based on the picture you include in your blog, that shape would mount nicely along the fenderwell where you have the 'reservoir' currently attached.
I'm going to use it in my Fiat conversion. As to the detailed info, thanks so much. It's very helpful. I've already started searching for my pump! :o)
Thanks again,
Ken
Thanks for the tip!
I did consider the schedule 40 pipe solution, it's a fairly common approach to the vacuum reservoir problem.
I agree that it would be better than my electronics enclosure. I used the enclosure more out of expediance than anything else.
Do you have any pictures of your conversion? I wouldn't mind hosting them here.
-Peter
Peter,
As to pics of my conversion, not yet. I'm still in the "gathering parts and ideas" mode. I have my ICE motor out and on the ground ready to make a trip to the scrap yard. I've found a used Forklift drive motor for my motor for $80. It should be here some time next week. Unfortunately, we're just starting a remodel that my wife insists must be done in time for Thanksgiving. So, all my free time is consumed for the time being.
As to the brake "vacuum" problem, one of my buddies, who builds showcars (on the level of Jessie James), told me how they deal with the power boosters. The answer is simple: they remove them. They simply remove the power booster and mount the master cyclinder directly to the firewall. He says that occasionally they have to modify the plunger rod that comes off the brake pedal but that they do it to all of his cars. (He is a neat-freak and hates the look of the power booster under the hood.) They even do this on the 500hp monsters they build. Yes, with manual brakes you have to push a little harder, but you don't have to worry about keeping a vacuum or the extra electrical drain or finding a vacuum pump. It also creates less clutter under the hood.
This sounds like the perfect solution for the EVer. In the case of the Fiat, it makes total sense. With disc brakes all-around, it already has great stopping power.
As to the Fiat master cyclinder setup, I don't really care for the separate bowl/master cyclinder configuration. On the same friend's suggestion, I'm going to replace it with one like off a Mustang II. These have the bowl and pump all in one cast iron configuration. One like this will also take up less space under the hood.
It's going to require a little work with the brake lines, but it should be pretty straight forward. I'm probably going to go that direction. My buddy may even have one from one of their showcar conversions that I can have. I like FREE!!! :)
Ken
In an EV you want as much braking action as you can get, especially if you're adding 1000 lbs of lead acid batteries in the trunk of your car.
I know some EV'ers who have choosen to remove the booster. To get enough braking action they often replace the master cylinder with one that's narrower and has longer travel for increased braking force. Some also upgrade the brakes with parts from a heavier car in the same line. Moving the lever point on the brake pedal for more force multiplication also helps.
Anyway, my feeling is that adding a vaccum pump and vacuum relay is no harder than any of the above upgrades. It's a question of personal preferences I guess.
Thanks,
-Peter
