This is a summary of a recent test of a Rosebud grate fitted to my 2 inch scale traction engine. I'm posting this note in case it might be of interest to others who either have such a grate or are thinking of fitting one. In summary, this grate is not as good as the previous bar grate in my engine.

I made the rosebud grate because my previous bar grate fell in half at the end of the rally at Guildford in July. This seemed like an opportunity to try out this new concept, and also seemed like less work than making a new bar grate (I have second thoughts on that point).

The grate is in 10 mm steel plate with 77 x 3/16 holes ( 7 x 11 x 0.4 inch pitch). These are countersunk with a centre drill. This provides an airflow area of about 12% of the grate area.

I ran the grate at the rally at Amberley last week, when it appeared to perform OK. I ran for 5 hours and covered 6.5 km. Pressure did tend to fall during a drive and I had to stop regularly to recover with the blower – until the last circuit of the day, when pressure held, and the relief valves were lifting as I came to the end of the run.

I ran the engine today in the back garden, driving a generator, intending to take some measurements of power and coal consumption, to calculate an efficiency, as I have done every September for several years. However, I was unable to sustain pressure even for a small load, and struggled over a couple of hours to achieve a sustained steady state. Always the pressure would fall to about 40 psi and needed the blower to recover. This is worse than the previous sessions with the old bar grate, when I could sustain the pressure at a load of 40 – 50 watts. I conclude that the draught from the running engine is not sufficient for this grate with its greater air resistance,

I could fit a smaller blast nozzle, but I see that as a backward step for engine efficiency, so I am resigned to making a new bar grate similar to the old one from stainless strips ( and 22 stainless spacers – probably less work than drilling 77 holes!).

Why then have some users found an improved performance with a Rosebud grate? I have a hypothesis. I suspect that for an engine which has a strong blast with two cylinders, a bar grate might pass too much air, and fitting a Rosebud can restrict the air and thereby allow the fire to get hotter. In the case of a single cylinder engine, like mine, where I had reduced the blast over the years to just balance the grate for a typical rally drive, the air restriction has the effect one might expect – a poorer fire.

I would be most interested in any comments.

Paul