Qi W: Could You Live With The Shame?

Qi W: Could You Live With The Shame?: Could You Live With The Shame?

Two Stroke Engine

A two-stroke, or two-cycle, engine is a type of internal combustion engine which completes a power cycle in only one crankshaft revolution and with two strokes, or up and down movements, of the piston in comparison to a "four-stroke engine", which uses four strokes. This is accomplished by the end of the combustion stroke and the beginning of the compression stroke happening simultaneously and performing the intake and exhaust (or scavenging) functions at the same time.

The engine motion is like this:

Two-stroke engines often provide high power-to-weight ratio, usually in a narrow range of rotational speeds called the "power band". Compared to 4-stroke engines, they have a greatly reduced number of moving parts, are more compact and significantly lighter.

The first commercial two-stroke engine involving in-cylinder compression is attributed to Scottish engineer Dugald Clerk, who in 1881 patented his design, his engine having a separate charging cylinder. The crankcase-scavenged engine, employing the area below the piston as a charging pump, is generally credited to Englishman Joseph Day.

VVT(Variable valve timing) engine

In internal combustion engines, variable valve timing (VVT) is the process of altering the timing of a valve lift event, and is often used to improve performance, fuel economy or emissions.

The valves within an internal combustion engine are used to control the flow of the intake and exhaust gases into and out of the combustion chamber. The timing, duration and lift of these valve events has a significant impact on engine performance. Without variable valve timing or variable valve lift, the valve timing must be the same for all engine speeds and conditions, therefore compromises are necessary. An engine equipped with a variable valve timing actuation system is freed from this constraint, allowing performance to be improved over the engine operating range.

Piston engines normally use valves which are driven by camshafts. The cams open the valves (lift) for a certain amount of time (duration) during each intake and exhaust cycle. The timing of the valve opening and closing is also important. The camshaft is driven by the crankshaft through timing belts, gears or chains.

An engine requires large amounts of air when operating at high speeds. However, the intake valves may close before enough air has entered each combustion chamber, reducing performance. On the other hand, if the camshaft keeps the valves open for longer periods of time, as with a racing cam, problems start to occur at the lower engine speeds. This will cause unburnt fuel to exit the engine since the valves are still open. This leads to lower engine performance and increased emissions.


Methods for implementing Variable Valve Control 

Cam switching

This method uses two cam profiles, with an actuator to swap between the profiles (usually at a specific engine speed). Cam switching can also provide variable valve lift and variable duration, however the adjustment is discrete rather than continuous.

The first production use of this system was Honda's VTEC, which uses a locking pin to engage the "high rpm" cam follower.

Cam phasing

Many production VVT systems are the cam phasing type, using a device known as a variator. This allows continuous adjustment of the cam timing (although many early systems only used discrete adjustment), however the duration and lift cannot be adjusted.

Oscillating Cam

These designs use an oscillating or rocking motion in a part cam lobe, which acts on a follower . This follower then opens and closes the valve. Some oscillating cam systems use a conventional cam lobe, while others use an eccentric cam lobe and a connecting rod. The principle is similar to steam engines, where the amount of steam entering the cylinder was regulated by the steam “cut-off” point.

The advantage of this design is that adjustment of lift and duration is continuous. However in these systems, lift is proportional to duration, so lift and duration cannot be separately adjusted.

The BMW (valvetronic), Nissan (VVEL), and Toyota (valvematic) oscillating cam systems act on the intake valves only.

Eccentric Cam Drive

Eccentric cam drive systems operates through an eccentric disc mechanism which slows and speeds up the angular speed of the cam lobe during its rotation. Arranging the lobe to slow during its open period is equivalent to lengthening its duration.

The advantage of this system is that duration can be varied independent of lift (however this system does not vary lift). The drawback is two eccentric drives and controllers are needed for each cylinder (one for the intake valves and one for the exhaust valves), which increases complexity and cost.

Three-Dimensional Cam Lobe

This system consists of a cam lobe that varies along its length (similar to a cone shape). One end of the cam lobe has a short duration/reduced lift profile, and the other end has a longer duration/greater lift profile. In between, the lobe provides a smooth transition between these two profiles. By shifting area of the cam lobe which is in contact with the follower, the lift and duration can be continuously altered. This is achieved by moving the camshaft axially (sliding it across the engine)a stationary follower is exposed to a varying lobe profile to produce different amounts of lift and duration. The downside to this arrangement is that the cam and follower profiles must be carefully designed to minimise contact stress (due to the varying profile).

Ferrari is commonly associated with this system, however it is unknown whether any production models to date have used this system.

Two Shaft Combined Cam Lobe Profile

This system is not known to be used in any production engines.

It consists of two (closely spaced) parallel camshafts, with a pivoting follower that spans both camshafts and is acted on by two lobes simultaneously. Each camshaft has a phasing mechanism which allows its angular position relative to the engine’s crankshaft to be adjusted. One lobe controls the opening of a valve and the other controls the closing of the same valve, therefore variable duration is achieved through the spacing of these two events.

The drawbacks to this design include:

• At long duration settings, one lobe may be starting to reduce its lift as the other is still increasing. This has the effect of lessening the overall lift and possibly causing dynamic problems. One company claims to have solved the uneven rate of opening of the valve problem to some extent thus allowing long duration at full lift.
• Size of the system, due to the parallel shafts, the larger followers etc.

Coaxial Two Shaft Combined Cam Lobe Profile

This system is not known to be used in any production engines.

The operating principle is that the one follower spans the pair of closely spaced lobes. Up to the angular limit of the nose radius the follower “sees” the combined surface of the two lobes as a continuous, smooth surface. When the lobes are exactly aligned the duration is at a minimum (and equal to that of each lobe alone) and when at the extreme extent of their misalignment the duration is at a maximum. The basic limitation of the scheme is that only a duration variation equal to that of the lobe nose true radius (in camshaft degrees or double this value in crankshaft degrees) is possible. In practice this type of variable cam has a maximum range of duration variation of about forty crankshaft degrees.

This is the principle behind what seems to be the very first variable cam suggestion appearing in the USPTO patent files in 1925 (1527456). The “Clemson Camshaft” is of this type.

Helical Camshaft

Also known as "Combined Two Shaft Coaxial Combined Profile with Helical Movement", this system is not known to be used in any production engines.

It has a similar principle to the previous type, and can use the same base duration lobe profile. However instead of rotation in a single plane, the adjustment is both axial and rotational giving a helical or three-dimensional aspect to its movement. This movement overcomes the restricted duration range in the previous type. The duration range is theoretically unlimited but typically would be of the order of one hundred crankshaft degrees, which is sufficient to cover most situations.

The cam is reportedly difficult and expensive to produce, requiring very accurate helical machining and careful assembly.

Camless engines

Engine designs which do not rely on a camshaft to operate the valves have greater flexibility in achieving variable valve timing and variable valve lift. However, there has not been a production camless engine released for road vehicles as yet.

Types of camless engines include:

• electro-mechanical (using electromagnets)
• hydraulic
• stepper motors
• pneumatic

• Exceptions being the Mazda Rotary Engine which has no valves pistons or cams. But is still an internal combustion gasoline engine, and the 600cc engine used in the Trabant from 1957-1989.

Video for VVT engine mechanism

Different types of gears

Planetary gearing is the most popular and affordable type of winch gear. The name comes from the design, which is similar to planets revolving around a sun. The winch uses a small planetary gear and a coaxial shaft capable of large reductions in a small space. This device is used for handling high bearing loads with smooth operation, and provides good resistance to torque loads. They are between worm and spur gear types for the amount of drivetrain friction and amp draw with a transfer efficiency of 65%.
Advantages：
1.Low cost
2.Low weight
3.Compact size
4.Good resistance to torque loads
5.Smooth operation
Disadvantages：
1.Brake required to hold load
2.Drum gets hot when spooling

Planetary Gear

Spur Gearing is a simple gear using two or more parallel shafts with gears operating in the same plane. Generally, there is one large gear on the motor shaft and a smaller gear on the winch shaft. Spur gears are the most efficient type of winch gear with a75% transfer efficiency. Spur gearing produces less friction and greater efficiency than planetary and worm gearing, but requires a braking system.
Advantages:
1.Most efficient
2.Fast
3.Do not need a strong break
4.Low internal gear friction
Disadvantages:
1.Not easily mounted to vehicles
2.Expensive
3.Not widely available on the market

Spur Gears

Wankel Engine

The Wankel engine is a type of internal combustion engine which uses a rotary design to convert pressure into a rotating motion instead of using reciprocating pistons. Its four-stroke cycle takes place in a space between the inside of an oval-like epitrochoid-shaped housing and a rotor that is similar in shape to a Reuleaux triangle but with sides that are somewhat flatter. This design delivers smooth high-rpm power from a compact size. It is the only internal combustion engine invented in the twentieth century to go into production. Since its introduction the engine has been commonly referred to as the rotary engine, though this name is also applied to several completely different designs.

The engine was invented by German engineer Felix Wankel. He received his first patent for the engine in 1929, began development in the early 1950s at NSU Motorenwerke AG (NSU), and completed a working prototype in 1957. NSU then licensed the concept to companies around the world, which have continued to improve the design. Because of their compact design, Wankel rotary engines have been installed in a variety of vehicles and devices such as automobiles (including racing cars), along with aircraft, go-karts, personal water craft, chain saws, and auxiliary power units. The most extensive automotive use of the Wankel engine has been by the Japanese company Mazda.