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