konceppt

i had done a search for gearbox oil and this was on page 2

Steve Whitehorn

iTrader: (13)

For those that are interested.

Taken from THE CAR BIBLES by CHRIS LONGHURST

The difference between 4-stroke and 2-stroke engines

First, some basic concepts. Well one basic concept really - the most common types of internal combustion engine and how they work. It's worth reading this bit first otherwise the whole section on octane later in the page will seem a bit odd. Almost every car sold today has a 4-stroke engine. So do a lot of motorbikes, lawnmowers, snowblowers and other mechanical equipment. But there are still a lot of 2-stroke engines about in smaller motorbikes, smaller lawnmowers, leaf-blowers, snowblowers and such.
The difference between the two engine types is the number of times the piston moves up and down in the cylinder for a single combustion cycle. A combustion cycle is the entire process of sucking fuel and air into the piston, igniting it and expelling the exhaust.
There are two other types of engines - rotary or wankel engines, only used by Mazda in their "R" sports cars, and diesel engines. As this page gets updated, those will be dealt with accordingly.

2-stroke engines

A 2-stroke engine is different from a 4-stroke engine in two basic ways. First, the combustion cycle is completed within a single piston stroke as oppose to two piston strokes, and second, the lubricating oil for the engine is mixed in with the petrol or fuel. In some cases, such as lawnmowers, you are expected to pre-mix the oil and petrol yourself in a container, then pour it into the fuel tank. In other cases, such as small motorbikes, the bike has a secondary oil tank that you fill with 2-stroke oil and then the engine has a small pump which mixes the oil and petrol together for you.
The simplicity of a 2-stroke engine lies in the reed valve and the design of the piston itself. The picture on the right shows a 4-stroke piston (left) and a 2-stroke piston (right). The 2-stroke piston is generally taller than the 4-stroke version, and it has two slots cut into one side of it. These slots, combined with the reed valve, are what make a 2-stroke engine work the way it does. The following animation shows a 2-stroke combustion cycle. (You'll need Apple Quicktime 7 for it to display properly as I have used the new H.264 codec for quality). As the piston (red) reaches the top of its stroke, the spark plug ignites the fuel-air-oil mixture. The piston begins to retreat. As it does, the slots cut into the piston on the right begin to align with the bypass port in the cylinder wall (the green oblong on the right). The receding piston pressurises the crank case which forces the reed or flapper valve (purple in this animation) to close, and at the same time forces the fuel-air-oil mixture already in the crankcase out through the piston slots and into the bypass port. This effectively routes the mixture up the side of the cylinder and squirts it into the combustion chamber above the piston, forcing the exhaust gas to expel through the green exhaust port on the left. Once the piston begins to advance again, it generates a vacuum in the crank case. The reed or flapper valve is sucked open and a fresh charge of fuel-air-oil mix is sucked into the crank case. When the piston reaches the top of its travel, the spark plug ignites the mixture and the cycle begins again.


For the same cylinder capacity, 2-stroke engines are typically more powerful than 4-stroke versions. The downside is the pollutants in the exhaust; because oil is mixed with the petrol, every 2-stroke engine expels burned oil with the exhaust. 2-stroke oils are typically designed to burn cleaner than their 4-stroke counterparts, but nevertheless, the 2-stroke engine can be a smoky beast. If, like me, you grew up somewhere in Europe where scooters were all the rage for teenagers, then the mere smell of 2-stroke exhaust can bring back fond memories. The other disadvantage of 2-stroke engines is that they are noisy compared to 4-stroke engines. Typically the noise is described as "buzzy".

4-stroke engines

4-stroke engines are typically much larger capacity than 2-stroke ones, and have a lot more complexity to them. Rather than relying on the simple mechanical concept of reed valves, 4-stroke engines typically have valves at the top of the combustion chamber. The simplest type has one intake and one exhaust valve. More complex engines have two of one and one of the other, or two of each. So when you see "16v" on the badge on the back of a car, it means it's a 4-cylinder engine with 4 valves per cylinder - two intake and two exhaust - thus 16 valves, or "16v". The valves are opened and closed by a rotating camshaft at the top of the engine. The camshaft is driven by either gears directly from the crank, or more commonly by a timing belt.



Because of the nature of 4-stroke engines, you won't often find a single-cylinder 4-stroke engine. They do exist in some off-road motorbikes but they have such a thump-thump-thump motion to them that they require some large balancing shafts or counterweights on the crank to try to make the ride smoother. They also take a little longer to start from cold because you need to crank the single piston at least twice before a combustion cycle can start. Any more than one piston and the engine gets a lot smoother, starts better, and is nowhere near as thumpy. That's one of the advantages of V-6 and V-8 engines. Apart from the increased capacity, more cylinders typically means a smoother engine because it will be more in balance.
Geek trivia: Mercedes-Benz needed to increase the performance of their diesel passenger cars back in the 70's as their market share in the US was increasing. As professionals with big V-8 luxury cars were trading them in for 2.4l diesels, the demand for performance had to be addressed. Mercedes did not want to retool their 114/115 series chassis and there wasn't enough room in the engine bay for a six cylinder diesel. There was, however, room for a straight-5. Benz engineers just hung another cylinder on the back of the 4 cyl block and presto! The five cylinder engine was born. This engine acquired a lot of status among the high line car owners. When Audi introduced the C2 series cars (the 5000 in America, the 100 in Europe) in 1976, they offered a 5-cylinder petrol engine too. It was basically a 1.8 litre 4-cylinder engine with an extra cylinder. That took it up to 2.0 litres but the fifth piston made such an enormous difference to the smoothness of the engine that it was often mistaken for a V6 or V8. Why only 5 cylinders instead of going for a V6? Partly for the same rationale as Mercedes (and it was a really tight fit) but primarily because Benz had made the straight-5 configuration fashionable. A straight-5 was also more fuel-efficient than a V6. It's also worth pointing out that nowadays, both Audi and VW have V5 engines with three cylinders in one bank and two in the other. Same smoothness, better gas-mileage.

Full link here with pics and animations

Car Bibles : The Fuel and Engine Bible

Steve

Leslie

Don't be rude about Triumphs, I love my old Bonny!

Les