Tuesday, 5 July 2016

The NSU Prinz engine



During the 1950’s there was a dramatic shift away from motorcycles to car ownership. This resulted in decreasing markets for motorcycle manufactures, which in the case of some companies meant that diversification into the four wheeled market was an obvious step.


One such manufacturer was NSU, who in 1955 received the directive to produce a small four-seater car. The specification of the car was for it to have a mass of approximately 500 kg and a speed of above 100 km/h. Another notable design constraint for NSU was that it would have to utilise its existing equipment and machinery that was developed for the production of motorcycles.
NSU Prinz powertrain


The engine for this new vehicle was to be a twin-cylinder four stroke. During early scheming a 400cc engine capacity was considered, however NSU settled on a 600cc unit. The engine would be air-cooled as NSU had extensive experience of this through its motorcycle manufacturing and also due to production cost reasons.


The engine, gears and differential for the new car would be housed in a single transversally positioned block in order to ensure a compact, sturdy unit with a favourable centre-of-gravity and low production cost. The clutch was mounted on the left of the crankshaft as was the blower wheel, which was attached to the clutch driving wheel. The gear drive was obtained by helical spur gears between the clutch and main bearing. On the other side of the crankshaft ‘Dyna’ starting equipment was mounted, which controls lighting whilst the starter also acted as a flywheel mass. Cooling air from the blower wheel was guided into a sheet metal sleeve which forced it around the cylinder block and cylinder head.

The twin cylinder unit utilised a common ignition source, so for this reason the crankshaft had only one throw and consisted of three drop-forged parts, i.e. the two crankpins and two bearing pins to the right and left and of the compact centre piece, which was developed as a counter-weight and flywheel mass. Following the initial machining and grinding of the crankpins, the crankshaft was assembled with a press-fit and the insertion of an intermediate ring, which allowed their separation (by pressure) in the event of repairs. The final grinding of the main pins was completed after the press-fit of the components.

Lubrication for the connecting rod bearings was carried through channels leading from the main bearings into the hollow crankpins whose ends were sealed by blanking plugs.  Besides the two main bearings, there was one step bearing on the side of the clutch and one on the side of the lighting attachment. The clutch and pinion were held on the left pin by a pair of needle roller bearings. This method of supporting the crankshaft proved very successful as it resulted in lengthened intervals between overhauls. The big-end bearings in the connecting rod were not of the split-type.  The bearings for the mains and big-ends were conventional tripartite components (supporting plate of steel, lead bronze and a third layer for the purpose of reinforcement). The rods with their built-in bearings are fitted before pressing together the crankshaft assembly. Oil flung from the big-end bearings was sufficient to lubricate the cylinder bores and gudgeon pins.

NSU Prinz engine layout

As mentioned previously, the complete housing of crankshaft, gears and differential was constructed of an aluminium alloy pressure die-casting with high degree of rigidity. Extensive ribbing of the block surfaces was used to evenly distribute stresses at locations such as the thrust blocks and where heavily loaded screws were inserted. The lower part of the housing also acts as an oil sump, with a capacity of approximately 2 litres.

The two cylinders and the channel for the valve drive gear was cast in a single piece. The cylinder bores were induction hardened and, in order to reduce wear of the cylinder bores, an alloy of grey-cast iron and chromium was used as the material for the cylinder blocks. Rid-shaped openings for the inflow of cool air were placed between the two barrels and between the housing for the camshaft drive. The cylinder had passages for both the pressurised oil feed and oil draining from the upper valve gear. The cylinder bore measured 76mm and with a 66mm stroke resulted in a cubic capacity of 598cc.

The cylinder head for the two cylinders was once again cast in a single piece of chill-cast aluminium. Pressure die-casting was not possible due to the complex shape of the ports. The combustion chamber was semi-circular, which allowed valves with large diameters to be fitted. The valves were fitted at an angle of 32° to the cylinder centre. Both the valve seats made from grey cast iron and bronze valve guides were shrunk into the head. The cylinder head had comprehensive finning on all surfaces including the centre section between the two combustion chambers. The inlet valve seats had hardened surfaces, whilst the exhaust valves were armoured with a special heat resisting steel alloy. Two valve springs were fitted to each valve.
NSU Prinz engine layout showing valve gear


Two cam ladders were fitted onto the cylinder head for the camshaft which was die-forged and case hardened. It was also hollow in order to provide lubrication to the camshaft bearings and cam tracks. The rockers were also case-hardened and fitted with a hardened chromium rubbing layer.
NSU cam drive layout

Probably the most technically interesting aspect of the engine was the drive methodology for the camshaft. On the side of the lighting attachment, the crankshaft was fitted with a control pinion which operates a control wheel on a short shaft. On the same shaft are two narrow eccentrics. Light connecting rods were utilised to transmit this rotary motion to similar eccentrics on a short shaft in the cylinder head, substantially in line with the camshaft. The weight of these ‘side rods’ was compensated by counterweights. In order to prevent distortions through heat expansion of the cylinder or cylinder head, the upper eccentric shaft runs on roller bearings in a separate housing. This housing is pivoted to the cylinder head to one side. A third thin connecting rod, non-rotating serves to maintain the true distance between the eccentric shafts in the crankcase and the cylinder head. The non-rotating intermediate rod was placed close to the rotating connecting rods and as a consequence is equally heated, while still keeping the necessary distance for the control gear, quite independent of the heat expansion of cylinder and cylinder head. Transmission from the upper eccentric shaft to the camshaft which, although placed in line are not necessarily coaxial, is obtained by a pressed-in pin on each of them and a trailing link connecting these pins. All supporting points of the valve gear were generously lubricated, which also acted to remove heat. This system of cam drive was first used by NSU on their Max motorcycle. It was found to require practically no maintenance whilst also ensuring high levels of rigidity between the crankshaft and camshaft. With this valve drive system NSU’s Sport-Max proved to be a highly successful 250cc racing machine which was even able to win the World Championship.
NSU Prinz ventilation system layout


The car was heated by fitting heat exchanges around the exhaust manifold. Intake air flowed through an oil-bath/ air filter to a Solex carburettor. An NSU developed diaphragm pump fed fuel into the carburettor.

The drive from the engine was transmitted via spur gears to the synchronized gears. For this reason the gearshafts are parallel to the crankshaft and rear axle-shafts. The output was transmitted from from the gears to the helical differential drive-wheel. Universal joints for the swinging rear axle shafts are fitted to the two outlets of the differential housing.

The engine unit was suspended in the vehicle using one rubber mounting to the front and two to the rear.
Complete NSU Prinz drive assembly

With this powertrain, the Prinz-4 weighed 565 kg and was capable of reaching 120 km/h. Fuel consumption was approximately 5.7 litres/100 km. In the Sports-Prinz, the vehicle weight was 555 kg, whilst a top speed of 133 km/h was achievable as a result of a more aerodynamic body shape.
NSU Prinz performance

Sunday, 12 June 2016

Sodium filled exhaust valves - not just a Norton issue

Recently I wanted to throw in a ‘curve ball’ type question to an interview for a young engineer to test their problem solving. The curve ball question I came up with related to the exhaust valve illustrated below.
Works Norton sodium filled valve

So this is no ordinary exhaust valve. 
It is a Works Norton Valve from one of their famous 500cc single cylinder machines. What is most notable is this is not a valve fitted to one of Norton’s manx production racers. The stem is the giveaway; and what a giveaway that is given that it is ½” in diameter!
Looking back now it is fair to say this is ghastly design. But this is where we need to put our problem solving hat on and consider why it has such a structure. The larger diameter is of course filled with sodium. Sodium is a group one metal which becomes liquid at relatively low temperatures. The liquid sodium can convect heat away from the valve head.
The exhaust valve is also noteworthy for its convex head. Once again this shape is chosen not to increase compression ratio (there are far easier ways to do this, e.g. material on the piston etc), but is instead incorporated to take more heat out of the combustion chamber.
So the convex head and large stem filled with sodium all points to increasing heat transfer away from the combustion chamber and most importantly away from the valve head itself. 
Why was this required?

Many far higher performance engines can operate perfectly satisfactorily with conventional solid exhaust valves.
Once again we need to look back at the period when the engine was racing; post WW2 when petrol supply was in short demand and resulted in low octane ‘pool’ type fuels. The lower octane fuels had a far greater tendency to result in engine ‘knock’. As we know there are a few mechanisms that cause uncontrolled combustion in a spark ignition engine (knock, pre-ignition), however one item that can be big influence is hot spots within the combustion chamber. It is no surprise that an exhaust valve is one of the hottest parts of any engine. 

So there we have it, on the very low quality petrol available post WW2 Norton tried all they could to prevent hot spots in the engine (hence heavily cooled exhaust valve) in order to prevent engine knock and allow them to use higher compression ratios and higher levels of ignition advance.
But the engineers of the time knew exactly this and were well aware that if different fuels were used, a sodium filled exhaust valve may not be required. In the words of Steve Lancefield when talking about Manx engines for 500cc racing cars which could run higher octane alcohol fuels having greater charge cooling characteristics:
“Yes, most of the Norton engines I prepare are fitted with ‘Sodium’ exhaust valves – it is open to question whether these are really necessary on such well-cooled engines using alcohol, particularly as very good results have been obtained with engines using non-sodium valves. However, for what it is worth, the sodium valve is a shade lighter in weight than its solid counterpart and the use of this type of valve is on this score alone worthy of consideration. When using hydrocarbon fuels, unhesitatingly – sodium-filled exhaust valves please!”
Sodium filled valves are however not just a remnant of engineering history.  Modern turbo gasoline direct injection (TGDI) engines with their very high specific outputs are also now requiring such technology once again to reduce valve head temperatures to prevent knock and allow higher compression ratios, higher boost pressures and more ignition advance. The images below details types of exhaust valves that can be used on engines and their influence on the valve surface temperature. The sodium filled valve (A) also utilise a hollow head filled with sodium to improve heat transfer – striking similarities to the issue and solution Norton came up with over 60 years previously! The sodium filled valve results in stronger valves (further from the fatigue line) with lower surface temperatures (better for knock) and also having a reduced weight compared to the solid equivalent.
Exhaust valve cross sections

Exhaust valve surface temperature and strength at peak power conditions in a modern TGDI engine


Wednesday, 1 June 2016

Ferrari 308GTB: Last throw of the 2 valve dice.



This blog relates to the P6 high performance camshafts that formed part of the Group 4 Specs Kit and were fitted to the Ferrari Type F106 engine. 



In the late 70s when ordering a new Ferrari 308GTB it was possible to specify an increased performance option for the car. This was commonly known as the ‘Sprint Pack’. It included Borgo high compression forged pistons, special camshafts, ANSI sports exhaust and alternative carburettor settings. These performance parts were derived from the Group 4 Specification Kit that had been developed by Ferrari for competition use.





It did not come cheap and added over £3000 to the overall cost of the car. If you consider that in 1979 a standard 308GTB would have cost a shade under £19,000 and at a cost of £24,560 you were well on the way towards the price of a Boxer. To be fair this figure also included the deep front skirt, 7.00J-front and 8.00J-rear Speedline magnesium alloy wheels and Pirelli P7 tyres. I suppose there was more to it than just bolting on a few goodies, apart from the additional engine work, the suspension had to be completely re-set to allow for the bigger wheels and tyres. 



As many will know, the Ferrari 308GTB was powered by a 3.0 litre Type F106 engine. This was a normally aspirated, 90 degree V8 with 4OHC and was S.A.E. rated at 255bhp. It was a 2valve engine with a bore of 81mm, stroke of 71mm and compression ratio of 8.8:1. The engine was originally designed as a competition unit with small capacity crankcase, dry-sump and scavenge pump. It had previously been fitted to the Dino GT4 in a wet-sump form, but only for the European Specification 308GTB was it dry-sump as originally intended.  It was also fitted to the 308GTS, but once again in a wet-sump form.




So to the camshafts.

The camshafts from the Group 4 Specification Kit are Ferrari designated P6.
The initial specified timing for these cams was:-         Inlet opens         48 Deg. BTDC
                                                                                                Inlet closes         62 Deg. ABDC
                                                                                                Exhaust opens   64 Deg. BBDC
                                                                                                Exhaust closes   44 Deg. ATDC

The maximum valve lift is 9.25mm for both cams, with a duration of 290 Degrees for the inlet and 288 for the exhaust. Valve overlap is therefore 92 Degrees. The above figures are based on a setting clearance of 0.5mm.

The Ferrari factory built LM308 competition car used P6 cams with the following specified timing:-



Inlet opens         51Deg. BTDC
Inlet closes         58Deg. ABDC
Exhaust opens   64 Deg. BBDC
Exhaust closes   44 Deg. ATDC
                                                                               
The above timing is used when the sprint pack is fitted and gives a duration of 289 Degrees for the inlet, 288 Degrees for the exhaust and a valve overlap of 95 Degrees when using a setting clearance of 0.50mm.

Clearly the inlet cam has been advanced 3-4 Degrees from the original P6 specification, the exhaust timing and valve lifts are as before. As to why this was done, one could surmise that advancing the inlet cam will pivot the torque curve to improve low speed and mid-range torque. The other consideration with advancing the inlet cam is that the resulting increased valve overlap is likely to make combustion stability at part load and idle conditions worse; as a result the carburation settings with such cams will be far more critical to ensure good engine running stability. This advancing of the inlet cam timings suggests that Ferrari realised that their P6 cam intake duration was a little on the long side; as such advancing the inlet cam may result in improved low speed torque with only very small penalties at peak power conditions. The result of this cam timing combination is a road car engine with an unusually large level of valve overlap; something that really was the last throw of the 2-valve engine development dice before the introduction of better breathing 4-valve engines with their potentially more modest cam timings.

As a comparison the standard 308GTB valve timing is:-  Inlet opens             30 Deg. BTDC
                                                                                                       Inlet closes             50 Deg. ABDC  
                                                                                                       Exhaust opens       36 Deg. BBDC
                                                                                                       Exhaust closes       28 Deg. ATDC

The maximum lift for the inlet cam is 9.00mm and 8.375mm for the exhaust. The above timings give a duration of 260 Degrees for the inlet, 244 Degrees for the exhaust and a valve overlap of 58 Degrees when using a setting clearance of 0.50mm.


Standard/LM308 valve timimgs


So it is clear that the standard 308GTB cam timings have a shorter duration than the P6 Le Mans specification. What is perhaps most notable in the comparison between the two, is the far higher valve overlap with the P6 Le Mans spec timings; this is due to both the longer duration and in the shifting of the inlet cam maximum opening point (MOP) to an earlier position. The earlier inlet MOP with the Le Mans spec P6 cams would typically suggest aiming towards lower speed torque, however when this phasing is combined with the much longer cam duration, an increase in cylinder filling/performance at the peak power engine speed will result. Based on the much lower valve overlap of the standard 308GTB cam timings, it would be anticipated that driveability at low loads would be improved and also suggests that engine setup (carburation, ignition timing etc.) would not be as critical. This would mean general carburettor settings could be used in production and calibration for each individual engine would not be required. 

Another noteworthy point concerning the use of P6 cams timed to Le Mans specification (as in the sprint pack), is that it will be fairly important that they are used in conjunction with the 9.5:1 high compression pistons that also form part of this kit. If standard lower compression ratio (8.8:1) pistons were used with the P6 Le Mans spec cams, combustion stability at part loads and idle may be worse due to higher levels of residual (exhaust) gas that will remain in the cylinder after the closing of the exhaust valves.



So what difference does this all make?
It is reputed that the sprint pack added 40bhp to the output of the engine, making it just shy of 300bhp. The exhaust note is completely different to that of the standard car, you would expect it to be louder with a sports exhaust fitted, but it is also noticeably ‘flatter’ in sound. When the timings and carburation have been set correctly, the car is perfectly usable on the road. The tick-over is both stable and reliable, with no tendency to stall when setting off. As with all 308’s it starts to pull strongly once 3000rpm is reached and builds up from that point on. There are no flat spots throughout the rev range, however at 5500rpm the engine takes another breath as it ‘comes on cam’ and launches itself towards the red-line at a vastly increased rate.

Carburettor settings.                         Weber 40 DCNF

                                                                Main jet               1.45
                                                                Air correction     1.70
                                                                Emulsion tube    F36
                                                                Slow running      0.50

Sorting out the carburation took a little time. The engine had been running  rich at the bottom end and needed no choke whatsoever to make it fire from cold. A slight misfire in this range became progressively worse as the engine warmed and became totally unacceptable when hot. Two reductions in the size of the slow running jets cleaned up the carburation and transformed the way it drove low down in the rev range.


  Before I finish I would like to leave you with a photograph of two all-time greats.


Niki Lauda & 308GTB at Fiorano