A 15-cwt. Van with an "Elastic" Engine
Page 10
Page 11
Page 12
If you've noticed an error in this article please click here to report it so we can fix it.
By .
L. J. COTTON, m.I.R.T.E.
THE unusual characteristics of the Trojan two-stroke petrol engine provide good torque over a wide speed range and give the vehicle an exceptionally good performance. By judicious use of the gears, this engine " elasticity " can be employed to obtain an economical fuel consumption when operating on local delivery work.
There are two pairs of working cylinders, each pair having a common combustion space. The mixture of petrol and air is drawn from the carburetter into two charging cylinders and is delivered to ports at the base of the working cylinders. A conventional compressor-type valve is interposed between the charging cylinders and the transfer ports.
The inlet and exhaust ports are controlled by the pistons, and to assist in scavenging and improve economy, the exhaust port is uncovered before the fresh charge enters the cylinder and is closed earlier than the transfer ports. This result is achieved by offsetting the cylinder-block axis from the crankshaft centre line, which, in conjunction with the cylinder layout, produces a differential piston action.
112 In the arrangement of the working cylinders, each cylinder of a pair is staggered, because the two connect. ing rods operate on a common journal, with the charging-cylinder connecting rod between them. The front charging cylinder works in conjunction with the rear pair of working cylinders, and Vice-versa. The two-throw two-bearing counterbalanced crankshaft is machined from a Meehanite iron casting.
The big-end bearing arrangement consists of a split phosphor-bronze sleeve which extends for the full width of the crankpin. It is lined with two white-metal. bearings separated by a circumferential gap which forms an oil groove. The bigends of the workingcylinder connecting rods Lre in contact with the outer diameter of the e v e. A solid white metal lined bearing carries the crankshaft at the front of the crankcase, whilst the rear bearing has flanges to control end-float of the shaft.
Cast-iron pistons are used in the charging cylinders to counterbalance the weight of the two aluminium pistons used in each pair of power cylinders Five " toe-in " ores-, sure rings are fitted, three above and two below the gudgeon pin, on each of the pistons of the power cylinders.
The power and charging cylinders are made from cast iron, the working cylinders having three-quarter length water jacketing. Aluminium heads are fitted to the power cylinders and to the air-cooled charging cylinders. Each pair of power cylinders shares a common head, with a sparking plug positioned over the inlet cylinder.
LubricatiOn System
Lubrication is provided by a gearPattern pump, which delivers oil to a full-flow filter. An external oil pipe connects the filter to a drilling leading to the front main bearing, and to an internal pipe supplying lubricant to the rear main bearing. The big-end bearings are fed through the crankshaft, whilst the distributor drive is lubricated through a metered supply, the overflow flooding the gears and bearings.
To reduce Maintenance of the ignition system, there are. a twopoint contact breaker, two condensers and two coils. Each pair of points is connected to a coil, so that the high-tension circuit is confined to the coil and sparking plugs, and the high-tension distributor is eliminated.
A three-speed unit-construction gearbox is hued, having synchronizing arrangements on the two higher ratios. First and reverse gears have straight teeth with sliding engagement. The gearbox is suspended from the frame, in conjunction with the engine and clutch, by rubber rings in shear.
The axle shafts of the three-quarter floating rear axle are splined at the inner ends, and the forged-steel hubs are fitted with a taper and key at the outer end. A four-pin differential is employed in the spiral-bevel assembly of the axle.
A rigid frame is formed of 10 S.W.G. channel section, with a box section at the highly stressed points and a cruCiform bracing at-the centre. The frame is spot ,yelded and riveted, and, although rigid, is
light.
made a check of weights before starting the trials, and found that a generous 17-cwt. payload of iron
-castings was being .carried. The
distributed weight on the rear axle was more than the advocated burden for the tyres, but withthe correct payload they would be within the prescribed limit.
Rapid Get-away
Leaving the Trojan works at Purley Way, Croydon, the test vehicle increased speed up the incline past Croydon Airport and continued to accelerate, according to the speedometer, to 45 m.p.h As this appeared faster than the road speed, I made a check and found that the instrument was reading 5 m.p.h. fast at 20 m.p.h. and 30 m.p.h. The results of the day's tests are modified to allow for this inaccuracy.
An attempt was
made. to climb the 1-in-41 gradient of Sticcombs Hill, from a standing start, but although the Trojan made a gal
lant effort, t h e engine revolutions became slower and slower until finally it stalled. A flying start was made from the roundabout, with bottom gear engaged and the engine turning at peak revolutions, but the " gradient was too severe for the fully laden machine. When the load was reduced to about 10 cwt. the vehicle made a straight climb. To expect a 1,186 c.c. engine to • propel over 2 tons up 1 in 4.1 was, perhaps, a little unreasonable.
After replacing the castings I drove to the foot of Bug Hill, an
incline of less severity Radiator temperature and ambient before moving away from rest were 198 degrees F. and 50 degrees F respectively. Low gear was employed for the climb, which has an average gradient of approximately I in 10, and the engine was putting hard for over three minutes. It was a severe test, and the water temperature rose to 207 degrees F There was a slight pull to the near side when the brakes were applied to restrain the speed to 30 m.p.h. for the descent of Westerham Hill. This could be attributed to high spots on the facings, and by the finish of the 70-mile test, all wheels were braking more evenly.
Consumption trials were made between the outskirts of Westerharn u3 and Riverhead. via Brasted, returning by the same route. The first test was made with the minimum of halts, and the eight miles were covered at an average speed of .29 m.p.h. There were five traffic-light or other delays during the trial, and the fuel consumption worked out to 25.6 m.p.g.
A repeat run was made, with four 15-second halts to every mile. Top gear was engaged as early as possible and the flexibility of the engine showed to advantage in the comparatively economical results. Deducting standing time, the course was covered at an average speed of 23 m.p.h., with a consumption rate of 18.2 m.p.g. These figures show that the Trojan has an excellent top-gear performance
Standing Start on 1 in 6 Retracing the outward course as far as Westerham Hill, I drove along the Pilgrim's Way to make a stopstart test on the 1-in-6 gradient by the church at the foot of Titsey Hill. This was a particularly difficult test, because the start is made on full lock as well as on a severe gradient.
The Trojan rose to the occasion and although there was little power in reserve for the first 100 yds., it made a clean climb at the first attempt. I took the radiator toptank temperature, after five minutes' stiff climbing in the indirect :ratios, and again it was over the 200 degrees F mark Just before reaching the branch road to Addington, there was a concrete streteh.of road ideally suited for the acceleration and braking trials, and although well above sea level, there was little wind to cause any variation in results.
30 m.p.h. Reached in 24 secs.
By using the gears, the vehicle could be taken swiftly off the mark, acceleration from 0-30 m.p.h requiring 24 secs. Low gear was used to move away from rest, with a quick change to the second ratio, which was held up to 20 m.p.h. before
engaging direct drive. A delivery van driver would probably adopt the same technique.
Top-gear trials demonstrated the engine flexibility, and full throttle opening at 8 m.p.h evinced no protest from the power unit. The transmission take-up was smooth, and there was no appreciable vibration. It took 24.4 secs. to reach 30 m.p.h. from a rolling start of 10 m.p.h. Just above 30 m.p.h. there was a slight vibration from the power unit, which could be detected by the .oscillation of the gear lever. • This la4 period was between 35 mph. and 38 m.p.h
Because of the uneven wear on the brake facings, there was a tendency to pull to the near side on emergency braking, and adjustment could have been made, but i preferred to test the vehicle as supplied This follows my rule that no adjustments, except those vital to road safety or to the life of the machine, are made during test.
On the dry concrete surface the
trials were made without wheel locking, and the Trojan was brought to rest in ;39 ft. from 30 mph. This was a highly satisfactory result, and subsequent tests made with light applications, showed that retardation was quite proportional to the pedal press tire.
With the facings thoroughly bedded down, and after the necessary resetting of the clearances, the maximum braking efficiency would most probably be improved.