Engines of Yore: Part 2.
Ariel Square-4. created 26 April 1992
by Dave Williams [email protected]
Much of the Ariel's mystique may have come from its relatively enormous
997cc displacement. In many ways, the Square-4 was two 500cc twins
mounted one behind the other, with all the usual weirdities of preWWII
British engineering. We'll be discussing the second generation "light
alloy" Square-4.
The crankcase followed normal practice, being vertically split aluminum halves. Unlike many of its contemporaries, the Ariel provided a wide flange at each half to minimize oil leakage. The crankshafts are assembled from a Z-shaped cranked section, a circular flywheel bolted to a central flange, and two bolt-on counterweights. The British were really into flywheels. The front and rear cranks were different, the rear having extended end shafts for clutch and magneto drives. The single camshaft sat in the middle of the crankcase and above the crankshafts. It was held by a plain bearing on the right (driven) end and a plain bearing on the left.
The engine used pressure lubrication. The left side of the cranks ran in ball bearings, the right side and rods in plain bearings. There were no center bearings, which was normal practice for the time. Oiling was via low volume plunger pump. A gauze filter about the size of a 35mm film can was provided for cleanliness.
The crankshafts were arranged 180 degrees out of phase to minimize vibration (which worked quite well, incidentally) and were phased by large gears on the left side. The engineers evidently didn't quite trust plain bearings, as this was the side they put the ball bearings in. Modern designers would have cut teeth in the flywheels since they nearly touched anyway, or used a full-circle flange on one of the rod throws and toothed that. The phasing gears were covered by another side plate, the rear of which mounted yet another ball bearing to stiffen the crank before the primary drive sprocket went on.
Connecting rods were aluminum with replaceable bearing shells. The rod bolts were quite modern looking with reduced shank diameters and precision-ground outside diameters to locate the big end caps. The British have never quite trusted screw threads, however. These fine bolts were finished off with castellated nuts and cotter pins. Pistons were conventional aluminum, dish top, 5.8:1 CR.
The Ariel's cylinders were all cast as one 4-cylinder block also providing the pushrod tubes. This was much different from usual practice, which provided individual cylinders on twins. The cylinder block was cast iron, as was the main head casting, which provided the four combustion chambers, fins, and the bottom half of the intake manifold. The aluminum rocker box set atop this, with a final aluminum rocker cover.
The overall design of the cylinder block and top end is difficult to justify by any rational means. It was surely expensive to build, and it is difficult to imagine a worse way of arranging a square four.
The main head casting was held down by only three bolts around each cylinder. The combustion chambers had no quench area. The intake air came straight down from above with an X shaped runner set cast right into the head, which surely warmed the inlet air more than necessary. The Ariel's engineers made the intake and exhaust valves the same size and mounted them straight up in the center of the chamber. The eight pushrods came up from between the cylinders, through the bottom head casting, and up into the light alloy rocker box. The rocker box also contained part of the intake tract, turning an abrupt 90 degrees and leading back to a single carburetor flange. Incoming air had to turn three 90 degree bends before it could get to a valve.
The valve gear is fairly ordinary shaft mounted rockers, except the Ariel designers designed the rockers for a 1:1 lift ratio. Pushrods were aluminum. The intake and exhaust lobes are identical, with lift at the valve was only 5/16". The cam timing was quite modest. Dual concentric valve springs were used with a 90 pound seat pressure, which is about the same pressure a 350 Chevy uses to control its much heavier valvetrain. I can't imagine why such stiff springs were used with such a mild cam profile, and I'd bet money the long, slender camshaft (unsupported in the middle, remember) would have flexed quite a lot.
Since the Square-4 couldn't suck any great amount of air in, its designers didn't worry unduly about getting air out. Grotty iron exhaust manifolds bolted to each side, and both cylinders shared a common exhaust pipe.
The Ariel Square Four produced 38 horsepower at 5,500 RPM from 997cc, not exactly sizzling even by the standards of its day. The square layout probably never caught on due to carburetion problems; the only other square fours I'm aware of are two strokes, which found the square layout to be an efficient means of using space. Yamaha and Honda have found the V-four to be easier to carburet and free of the hassles of double crankshafts and their phasing gears.
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