Pent Roof (or Crescent) Combustion Chambers - Advantages

My first car was a Ford Prefect bought for £2.00 from a car breaker and used by me and my mate to charge around a local farmer’s field. It was a 4 cylinder side valve engine and we learned a lot from the constant stripping down to keep it going.

That was in the late fifties and I graduated to an old BSA 250cc motor cycle, a side valve machine, and once again rode around the old farmer’s field; thus began my age-long love of motorcycles.

Around then my Dad bought a Morris Minor; a 998 CC side valve engine with the L-shape combustion chamber, and I was able to carry out the maintenance on it.

Soon OHV engines were the vogue, then the OHC, all with various combustion chambers and eventually the pent roof combustion chamber evolved.

This is an article on Mechanical Engineering/Machine Design and in particular the benefits of pent roof combustion chambers. We begin then with an overview of the IC engine, and then move onto the different designs of combustion chambers with the benefits of pent roof combustion chambers.

The model T Ford was one of the first mass produced automobiles to be powered by an internal combustion side valve engine built by the Ford Motor Company beginning in1908.( A well known slogan – any color as long as it is black?)

From 1932 they used a modernized version of this engine in their model Y autos. As an aside, the electrics were powered by a dynamo and the windscreen wipers operated by a vacuum from the carburetor manifold. I remember these well on Dad's 1950 Morris Minor; the wipers used to slow down as the engine labored up hills!

On the motorcycle front Harley Davidson used a side valve engine as early as 1919. The Davidson partner originally came from quite close to my village, and every year we have a Harley Davidson Rally with Bikers from all over the world attending – a great day out in the Highlands of Scotland!

Anyway, to continue, overhead valves and overhead cams (OHV and OHC) had been around for a while as well, but not exploited commercially until 1927 when Standard Motor Company introduced their V6 OHV engines, followed in 1949 by Oldsmobile. They produced an OHV engine operated by pushrods and fitted to their Rocket V8, but before that Norton Motor Cycle Company had produced a single OHC engine between 1925/26, that won the 500cc Isle of Man TT race in 1926.

The pent roof combustion chamber was reputedly used first in the engine of the 1904 Welch Model 4.0 built by The Welch Motor Co., Pontiac, Mi.

Pent roof combustion chambers were also used by Chrysler between 1951 and 1958 when it was discontinued due to high costs. Honda Motorcycles were using the pent roof combustion chambers around 1962, and I well remember Mike Hailwood, their star rider and world champ pushing a 500cc Honda out onto the start grid at the Ulster Grand Prix. We were all looking out for a “Honda Six” at the time and this machine had six exhaust megaphones poking out from the engine. Mike sort of smiled at the cluster of photographers and dolly-birds around him, reached down, and removed two dummy exhausts – yep it was still the four cylinder model!

A 1966 version of the Honda 500cc ridden by Mike is shown below;

Anyway, this was probably the first of the pent roof multi-valve engines produced by Honda and it blew all the opposition away in TT racing: the emerging Yamaha and Suzuki two-strokes, even our old Irish current record holder, a 7R AJS rode by local hero Dick Crieth.

There have been numerous combustion chamber designs of various shapes produced to achieve the optimum combustion efficiency of the fuel. The combustion chamber consists of upper and lower components; the upper being the recess into the cylinder head, the lower consisting of the piston crown and upper cylinder.

Four of the more popular combustion chambers are examined below with simple sketches, please click on the images to enlarge:

• Hemispherical

Symmetrical design, valves equally spaced with flat piston crown. This was the nearest to the current pent roof design, incorporating a hemispherical shape cast into the cylinder head and was considered a compromise between manufacturing cost and engine efficiency. These normally have a central spark plug between the equally spaced valves.

• Wedge

Asymmetrical design with valves spaced off-centre. Piston crown flat. This is one of the oldest designs and was used at one time or another by nearly all the major manufacturers of IC engines. The spark plug is located at the thick end of the wedge, with the valves each side at the same angle as the wedge top edge.

• Bowl in Piston or Cup/Bowl
  

  click to enlarge

This is a symmetrical combustion chamber design with the valves equally spaced and vertical; the piston being recessed to provide the combustion chamber shape against the flat cylinder head. Favored more by diesel engine manufacturers it produced a very turbulent flow of combustion mix and was noisy.

• Pent Roof or Crescent

This is a combustion chamber of symmetrical design; multi- valves equi-spaced over crescent shape and at an angle, the piston crown having a matching raised shape.

Nowadays almost all IC engines have some variation of a pent roof combustion chamber. This produces a high volumetric efficiency having at least two exhaust and two inlet valves/cylinder, driven by a single OHC or double OHC. The pent roof combustion chamber also reduces exhaust emissions, but although a high flame speed induces more power, this can also produce excessive levels of NOx’s.

Reference Webs:

1. docstore: Combustion chamber shapes.
2. americanautos: First pent roof combustion chamber used?
3. secondchancegarage: Hemispherical combustion chambers.
4. fordscorpio: Pent roof designs.