[Kartbuilding] Engines section of your website.

[Stephen Burke]

Hi,

Thanks very much for reading the article on the engines, and also for 
providing some feedback. I have added your ammendments to the 
kartbuilding.net website. That article I originally wrote was 9 years ago, 
so it might well have been a little in-accurate.

Thanks again, and if you find any other typos or mistakes, just drop me an 
email.

Best of luck,
-steve


On Mon, 15 Jun 2009, Rhys Firth wrote:

>
> I was browsing around looking for ideas on building a go-kart for offroad mud and track use with an 850cc Mini motor and was reading your site. I notice your information on valve trains for motors is inaccurate. I have altered things below to introduce the pushord operation system, since only old 40's and earlier engines have valves besdie the cylinder, everything post 1940's has their valves above the cylinder, it's just the pushrods beside the cylinders
>
>
> The following:
>
> Over-head Valves [OHV]
>
>
>  This is where there are
> valves that open and close directly above the rising piston. These
> valves are operated via a timing chain and rockers all mounted above
> the piston. This setup is apposed to the valves moving up and down
> beside the piston, although this method is more direct not requiring
> rockers or timing chains as they can be operated directly via cam's
> from the crankcase, it limits the speed/ revs the engine can do.
>
>
>   Left -
>
>  OHV
>
>  Right - Conventional Valves
>
>
>  Double Over-head Valves [DOHV]
>
>
>  This is a system the same as above only instead of 1 valve for the inlet
>
>  and 1 for the outlet (exhaust) there are 2 for the intake and 2 valves for the
>
>  outlet. The theory is that the faster you can get the fuel into the combustion
>
>  chamber and the faster you can get rid of the exhaust the faster your engine
>
>  will run. With this dohv system there are 4 valves per piston. So if you have
>
>  4 pistons you have 16 valves."
>
>
> should read:
> Pushrod Operated Valves
> This is where a short timing chain turns a camshaft mounted at the base of the cylinders, elliptical (oval or pear shaped) lobes on the cam push a rod up alongside the cylinder where it causes a rocker to swivel and push down on the valve, opening it to allow air/fuel mix into or exhaust gasses out of the cylinder. These limit the revolutions the engines are cabable of due to the rods having a regrettable tendacy to bend when the revs rise and the inertia of the rocers and valves becomes to great for the pushrods to move in sufficient time to match the rotation of the camshaft lobes.
>
> Over-head Valves [OHV]
>
>
>  This is where there are
> valves operated via a timing chain turning a camshaft and rockers all mounted above
> the piston. This method has 2 versions, Rocker activated, there the camshaft lobes push directly on the rockers dicarding the pushrods used in the previous method discussed, or Bucket valves, where the camshaft is directly above the valve stems and pushes straight down on the valve to open them. Rocker activated OHV engines can reach higher RPM than Pushrod engines, but the rocker inertia is still present and excessive RPMs can cause a rocker to break. Bucket style valves have technical challenges to do with a rotating cam placing sideways stresses on the valve stems which renders them more expensive and more challenging to engineer and as such are mostly found on high performance racing and sports car motors, but allow engines higher revolutions than rocker systems.
>
>
>
>
>
>
>  Double Over-head Valves [DOHV]
>
>
>  This is a system the same as above only instead of 1 camshaft with different lobes on the same shaft operating the intake and outlet vbalves, there are 2 camshafts 1 for the intake and 1 for the
>
>  outlet. Pushrod and OHV engines are typically 2-valve-per-cylinder engines, some rare OHV engines are 3-valves-per-cylinder engines, this is to do with the need to fit both intake and outlet lobes on the same camshaft. DOHV systems allow more valves to be fitted into the system by doubling the number of shafts. The increased number of valves means more area is provided when the valves are opened for the fuel/air mixture to enter through and the exhaust gasses to exit through, increasing the efficiency of the engine and the amount of power (Hp or KW) to be created from a given CC size engine.
> However, this is not always so, there are a number of 2-valve-per-cylinder Rocker based DOHV engines, where the single intake and exhaust valves for each cylinder are run off different camshafts, this is so each camshaft is running against inertia values which are lowered by having to run only half the number of rockers, this allows slightly higher RPMs than 2-valves-per-cylinder OHV engines.
>
>
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