4 SRKT Engine

Having already successfully provided electric motors for BR (SR) units, English Electric diesel engines ,manufactured by the English Electric company of the United Kingdom, were the preferred choice for the new DEMU stock. 

The English electric range of engines were derived from the "K type" engine (10" bore x 12" stroke), developed in the 1930s. These diesel engines were marketed under the English Electric name into the 1960s and later under the name Ruston-Paxman.

There has been some discussion about the relatively low powered engine provided for these units and indeed, this low power led to the units having to be fitted with turbochargers and on some routes, being reduced to two carriages. It appears that the initial decision was based on financial constraints and was the smallest engine that would power the unit sufficiently to improve on the previous steam engines timetable preformances. In that respect it was a success. 

The engines fitted to DEMU stock were the Mark II "RK" engine, which had 4-valve cylinder heads. They ran at 750-900 rpm, and were available as discussed with turbochargers.

The relatively slow revs of the engine are from where this stock gets the nickname "Thumper".

4srkt therefore stands for;

4 = number of cylinders
s = fitted with an exhaust supercharger
rk = Mark II "RK" engine
t = traction, M was used for Marine use

The diesel engine powered two English electric EE507D motors which produced 500bhp at 850rpm when fitted with the Napier MS 100 turbocharger. 

The engines (for the Hastings and Hampshire units) had a three point mounting. This was changed however, for the Oxted units, to a 4 point mounting with the fan being mounted on rubber pads to reduce vibration (Spoil sports).

Following the success of the units, in 1958 they were converted to three carriages with the insertion of a Trailer second carriage; this led to the engines being up rated to 600bhp with the replacement of the Napier MS100 turbo with the MS200 model. Some adaption of the fuel delivery system was also required. 

In later years, some of the EE507D engines; particularly those fitted to the 1200 series "Tadpole" units , used on the Reading - Tonbridge line, were replaced by the EE507E unit. This design of engine had roller bearing suspension, as opposed to the oil lubricating system of the earlier types. This was deemed necessary because of the high mileages accrued by these units. 

The English electric company was eventually merged with GEC in 1968, and GEC, having recently merged with Associated Electrical Industries (AEI), merged with EE; the former being the dominant partner and the English Electric name was dropped.