DAIMLER BUILDS A POPPET-VALVE POWER UNIT
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Full Particulars of a New and Interesting Power Unit to be Produced in Addition to the Company's Famous Sleeve-valve Type of Engine Which Has Won a Big Reputation
REALIZING that certain operators are more familiar with the poppet-valve type of engine, the Daimler Co., Ltd., of Coventry, is now producing a power unit of this type. It must be understood that this innovation is intended in no way to interrupt the flow of sleeve-valve engines, which still forms part of the Daimler Co.'s programme. Each type of engine is not only built to the high standards in quality of workmanship and materials for which all Daimler products have
for many years been renowned, but they also represent the latest principles of design and construction.
As will be seen from the accompanying illustrations, the exterior of the unit presents a clean and workmanlike appearance, and upon more detailed analysis it is clear that the whole design has been evolved to suit the at-
c42 duous conditions of service which is the lot of the majority of coach and bus power units. An interesting point is that the new engine is so proportioned that it will fit conveniently into the normal Daimler chassis without alteration and with little or no difference in power characteristics.
The crankcase is, of course, an aluminium casting which, with its boxgirder form of construction, makes a thoroughly rigid unit to carry the seven-bearing crankshaft. The sump
flange is located well below the centre line of the crankshaft and, being stiffly webbed transversely, avoids any possibility of " weaving " when the engine is developing its maximum torque.
Vollowing modern practice, the camshaft and auxiliary drives are arranged at the rear of the unit, the former being effected by a silent chain, whilst the latter obtains its drive by means of n triple-roller chain. It is needless to say that all the components which require periodic attention are grouped on the near side of the engine (only the
starter is located on the off side), so that the unit can be incorporated quite simply in fonward-control vehicles.
The crankshaft itself, although embodying journals of large diameter, is relatively light in weight because it is bored through the journals. As might be expected, the main bearings are supplied with oil from a full-pressure luhri
• cation system, which is extended to the crankpins by internally drilled passages.
The oiling system is really very comprehensive, for the osual Daimler feeture of an incorporated cooling arrangement is also included in this unit. Actually the pump is submerged in the sump and delivers oil to a gallery bolted to the main-bearing caps, a lead being taken from the gallery to a pressure-release valve, which merely blows back into the sump. A second pump circulates the lubricant through a special-section of the radiator (thereby forming a cooling device), the whole of this supply being taken via an AutoMean Mter bolted to the crankcase.
'Particular attention attaches to the design" of thd eareslaaft; special cam profiles have _been evolved which allow much greater tolerance in tappet adjustment than the total clearance now in comma)) use. Indeed, Daimler poppet valves can, it is claimed, be set at any clearance from .06 in. to .075 in. instead of the usual 3 thousandths to 5 thousandths of an inch, without noise becoming in any way noticeable.
We have actually heard an engine embodying such large clearances running under test conditions, and we were impressed by the extreme mechanical silence. From extensive tests carried out by the Daimler Co. this silence is maintained over long periods, the use of extremely hard valve guides, valve stems, etc., makings large contribution to this desirable state of affairs.
AnOther point worthy of note is that the push-rods do not vary the valve. rocker clearances when subjected, to any marked extent, to the heated conditions of running, and, in consequence, it will he realized that even under the maximum variation of a few thousandths of an inch little or no effect will be apparent in the running of the engine.
Particular interest also attaches to the shape and disposition of the combustion chambers. The valves are vertically disposed in the detachable cylinder head and are operated by rockers in the usual manner. At the side of the combustion chambers recesses are formed into which the sparking plugs are fitted. This form of chamber has been found to give extraordinarily efficient results, and at the same time pernuts the engine to turn over at low rates of speed without misfiring.
The inlet manifold is integrally formed on the off side of the cylinder head ; it is of rectangular formation and communicates with the carburetter— centrally mounted on the near side of the unit, below the exhaust manifold
by meaus of a passage cared in the cylinder block.
The carburetter is a Solex of the horizontal type and a hot-spot is formed at the junction of the intake port with the cylinder block. Petrol is supplied by a mechanical pump driven from the camshaft.
In the interests of general accessibility the ignition distributor is situ ated high up in relation to the engine, and the leads are short and neatly arranged. The drive for the distributor is obtained through the medium of spiral gears from the camshaft.
Although there are actually four engine feet the effective mounting in the chassis is of the three-point type, because the two front feet are situated fairly close together. The rear mounting is taken from the timing case.