[David Paule]

On a whim, we pulled the oil pump out of the box recently and took a look at it. I had previously disassembled it, so we weren’t able to examine it all together at this stage. We were curious, since the Anzani uses a pulsator tube instead of an oil pressure gauge, and therefore probably has low oil pressure compared to today’s engines, if we could get an idea of the volume of oil that is being circulated. I had thought that the Anzani’s oil flow was characterized by low pressure and high flow rate. Apparently it’s both low pressure and low flow rate. I think it was Craig who pointed out that the flow rate had to be low, since the oil system is a dead-loss system, to allow sufficient range for the aircraft. Roger said that the flow rate on modern engines is hundreds of times larger than this.

Here's a discussion about this.

It's worth bearing in mind that we didn't have the oil pump piston return spring out of the box of parts to assess.

Craig wrote -

Oil pump thoughts -
I think I have figured out how the oil pump works, and it isn’t pretty. The pump piston is driven in its cylinder by the pump cam. The cylinder has two ports: the output port that has a ball check valve, and the inlet port. The inlet port has no check valve and is always open to the oil filled compartment in the oil pump housing. As the piston reciprocates in the cylinder, the oil is forced in and out the inlet port. Because the inlet port is on the small side (about .16 inch or 4 mm dia.), the oil in the cylinder experiences pressure oscillations. The pump depends on the positive pressure pulses being high enough to unseat the ball check valve and squirt a little oil past it. The designer probably gave it a cam that rapidly drives the piston up and slowly withdraws it so as to make it more efficient.

The advantage of this design is that it only uses one check valve. The disadvantage is that its output is highly dependent on the viscosity of the oil. A low viscosity oil will just squirt out the inlet hole without generating sufficient pressure to unseat the check-valve. With the total loss oil system you don’t have to worry about the oil getting hot except on a hot day. Then the lower oil viscosity will result in lower flow just when you wanted more.
The real disaster, however, comes in the modern day when people like us decide to use modern oil. Modern synthetic oil has much better lubricity and the lower viscosity buys you…oops. The lower viscosity buys you no oil flow and a seized engine.

In order to use modern oil in the Anzani engine the oil pump has to be modified with a second check valve. I would hesitate to even use a 90 weight synthetic or modern castor oil because of the uncertainty in the viscosity of the castor oil used in 1918. Besides, we don’t know what gave these engines their poor reputations. The good news is I think we can easily modify the pump to take a second check valve.

Dave responded -

I'm not sure this is the way it actually operates.

I think that the intake port gets sealed off by the pump piston as it passes past the holes. So from that point on, there's an oil charge that's going to have to get past the outlet check valve because there's no other place for it to go. If this is correct, and I'm not looking at the hardware at the moment, just a sketch of it, it's a positive displacement two-cycle pump. The inlet "valve" is probably akin to a two-stroke motor valve, although I confess to being even more ignorant about those than I am about this. I think I've seen that sort of thing on model airplane engines when I was a kid.

Here's how it works, step by step. On the opening stroke, as the piston retracts, first it leaves a vacuum in the chamber when the check valve closes and seals. Then it slides past the intake orifice and that vacuum, plus the ready supply of oil with some head (a foot or two), fills the chamber. Then on the compression stroke, the piston pushes excess oil out the intake holes until the holes are sealed off. From that point to the end of the compression stroke, all the oil in the chamber is being pushed out the pump.

This depends on the stroke being reasonably large, but except for the loss of oil past the piston (which doesn't have sealing rings, so there might be some loss there) it supplies the same amount of oil regardless of viscosity. Perhaps more when it's thin than when it's thick, if the thick viscosity has difficulty filling the chamber. Thick oil would stress the pump more, obviously.

Proper setting up of the pump would have the piston opening no farther than the extent of the holes. Ideally, it should compress as close to the check valve or end of the chamber as possible.

Craig replied –

With regard to the oil pump, that is how I first thought it operated. However, I didn't like the idea that the piston, after giving its shot of oil, had to retract without the port being initially open. This would require the oil in the chamber to flash to vapor. It would also require the piston return spring to be strong enough to do it. Otherwise the piston/cam follower would just hang in the up position. The answer to this question is easily found by screwing the pump fully into its chamber and looking to see whether the piston travels beyond the port.

As to the crankcase compression idea, two-strokes have a valve at the inlet to the crankcase. It is either reed valves or a disc valve, or at least a piston port. Maybe not a check valve but at least something to allow the gas to be compressed. The oil pump has none (I think).


Roger added -

Well, Craig, Dave......those are interesting thoughts on the oil pump. I can see that moving oil via a pressure pulse instead of positive mechanical pressure keeps the pressure down and also avoids any potential dangers that are always possible with positive pressure pumps....not that I can recall ever seeing engine damage done by hydraulic "overpressure" from the oil pump, but in the early days they might have worried about it happening.

Maybe that is why so many of the oil systems we see on early engines were fairly low pressure. I've wondered about that. Surely they knew how to make the pressure higher if they had wanted to.

Here's an interesting aside: Yanmar is a large manufacturer of high quality diesel engines, and they still believe in low pressure for their diesel motors. Their engines have a direct drive positive pressure oil pump with no low pressure shutoff and a typical idle operating pressure of less than 5 psi. However, there are two different high pressure shut-offs that kick in about 35 to 40 psi.

The two stroke engine description should include one other pressure cycle. On a two stroke engine the volume of the crankcase is used to store a charge pulled in as the piston goes up and the pressure in the crankcase goes down.

After being pulled into the crankcase, the charge is compressed by the descending piston and also swept by the crankshaft counterweights - both of which move the charge in the direction of the transfer ports that feed it onto the top of the now-rising piston.

Just for grins, I think we ought the lift and duration of the oil pump cam. That would shed some light on which philosophy they used for the oil pump - was it pressure pulse or positive pressure.

Later, David wrote -

Got to thinking that if the piston is 1/4 inch diameter, and there's 20 psi of oil in the oil pump housing there (much more head than is likely, plus sea level atmosphere on it, and a vacuum on the other end of the piston) there's only about one pound of unbalanced force trying to prevent the piston from opening up. That's not a huge spring. And it would be a static preload on the piston (which may be why there was wear on the cam and lifer, in spite of being immersed in oil). That is, at the extreme open position the preload would be about a pound, and the cam pushes against the spring from there.

Then Craig -

Dave, You're right. I had also come to the conclusion that it wouldn't take much to vaporize the oil. That would be a unique way to operate a pump and now I am really curious.