ENGINE BASICS

source: motorcycleuniversity Moto U Engine Types: In Depth

We should begin this discussion by saying that no single engine type is superior to any other, however, different engines do perform differently, and some engines are better choices for different riding styles. A person riding a bike in the dirt may favor sharp bursts of acceleration while a person riding interstates with a passenger and luggage may desire smoothness and passing power at speed.

We will explain engines from broad to more specific.

All engines use “The Otto Cycle” or “Four Stroke” which describes the process of burning fuel to create rotary power within an internal combustion engine (as opposed to an external combustion engine, i.e. a steam locomotive). The four parts of the Four Stroke / Otto Cycle are:
1 Intake
2 Compression
3 Ignition
4 Exhaust

Also known in fun as: “Suck-Squeeze-Bang-Blow”

There are two major types of gasoline engines: Two-Stroke and Four-Stroke motors. The number of strokes defines how many times the piston changes direction in order to complete a power cycle.

Four Stroke Engines

On a Four-Stroke, the intake and exhaust cycles are controlled and kept mostly separate by valves. Four-stroke motors are more complex and heavier, but have better fuel economy and lower emissions. Four-stroke motors are almost universal in road-going motorcycles. For a given displacement, they are less powerful than Two-Stroke engines of equal displacement because they ignite the fuel-air mixture every other time the piston comes upward.

Two-Stroke Engines

Two-Stroke engines have been almost completely banned from public life. Most Two-Stroke motorcycles available today are either collectors items, or off-road racing bikes. They are still popular for racing because they are powerful, light-weight, mechanically simple, and cheap to maintain. They are more powerful for a given displacement because they ignite the fuel-air mixture (i.e. create power) each time the piston moves upward.On a Two-Stroke, the intake and exhaust cycles are not mechanically separate, which causes some unburned fuel to escape into the atmosphere, creating pollution (you can see a blue-gray tint in the exhaust of Two-Strokes). These engines are no longer allowed on most western roads, and are increasingly rare for dirt riding (but still in action). A Two- Stroke engine makes a distinctive “ring-ding-ding-ding” sound when it is revved up.

Two-Stroke engines also require lubricating oil in the fuel, creating additional pollution. Unfortunately, these chemicals create smog.

Displacement

Displacement is the volume of the cylinder measured when the piston is at the bottom of its stroke, and is usually expressed in Cubic Centimeters. Some American manufacturers describe the displacement of their engines in Cubic Inches. Displacement is a good predictor of performance. Large- displacement engines generally have more power than smaller-displacement engines. Therefore, most beginner bikes have smaller displacement engines.

Number of Cylinders

Generally, an engine with more pistons for a given displacement will have more power than an engine with fewer cylinders. Why? Surface area: the relationship between the volume of a cylinder and the diameter of the cylinder is such that if you divide one cylinder into two cylinders of equal volume and similar proportions, you actually increase the surface area of the pistons. This means that the burning fuel-air mixture has more surface area to push on during the ignition cycle, therefore, more power can be extracted. Why don't engines have eleven-teen cylinders? Because additional weight and mechanical friction eventually cancel out improvements in power output.

Most street motorcycles in the US market have engines of two or four cylinders. Production motorcycles have been made with one, two, three, four, and six cylinders. Worldwide, however, the most popular engine type is the small-displacement, single-cylinder because they are cheap to produce, easy to maintain, and thrifty on fuel.

In-Line, Vee, and Boxer.

In-Line arrangements have the cylinders parallel to each other, while Vee and Boxer engines have cylinders in two banks at an angle to each other.

An in-line engine has all of its cylinders mounted on a common plane. In-line engines can have any number of cylinders, with two, three and four being most common. There are six cylinder engines in large touring bikes.

A Vee engine has its cylinder banks mounted at an angle to one another yet acting on a common crankshaft. The V-Angle of the cylinder banks has a lot to do with the “character” of an engine, specifically regarding vibration.

Boxer engines are essentially a Vee engine, but the cylinder banks are 180 degrees opposite one another. A Vee or Boxer engine can have two, four, or six cylinders.

Engines can be designed for either transverse mounting or longitudinal mounting. Transverse mounted means that the crankshaft is perpendicular to the direction of travel. Longitudinal means the crankshaft is parallel to the direction of travel. Most motorcycles use the transverse-mounting arrangement for several reasons, but most importantly the rotational axis of the crankshaft, transmission, and rear wheel are all aligned in the same direction making it mechanically easier to move power through the drive train.

Final Drives

The final drive is the mechanical means by which the rotation of the engine is transmitted to the rear wheel where it drives the motorcycle forward, or how the engine is connected to the rear wheel.

Chains

The most common method is chain and sprockets. They are relatively inexpensive and quite reliable. By replacing components, they can also be used to alter the final drive ratio, i.e. how many RPM the engine spins for a given road speed. The downsides are that they require frequent maintenance, are consumable, and that the components are exposed to the world: rain, sand, road debris and road salt (if you live where it snows). These elements can cause a chain and sprockets to degrade and need replacement over time. Chain and sprockets are also sensitive to alignment, and can be noisy, but modern designs have reduced this problem, and chains are still the most common type.

Belts

Not as common, but mechanically similar to a chain and sprockets are belt drive. The belts are made of rubber and reinforced with space-age fiber such as kevlar. Belt drive has all the advantages of a chain, yet is completely maintenance free (i.e. no adjustment or lubrication ever) and nearly silent.

Shaft Drive

Drive shaft setups generally never need adjusting, they are very quiet, and are generally good for the life of the bike. The downsides are that if they do break, they are expensive to fix, and they are slightly less efficient than a chain.

Fuel Delivery: Carbs vs. EFI

In an engine, the fuel and air are combined to create a combustible mixture for the engine to burn. There are only two kinds of devices that do this: Carburetors, and Electronic Fuel Injection, commonly abbreviated as “EFI”.

Carburetors are mechanical devices which meter fuel and air and mix them together into a mist as the air is sucked into the engine. They utilize the Bernouli effect to draw fuel through a series of tubes and metering orifices called Jets. This is the same way a perfume mister or an airbrush works. Carburetors are “old school” and many people understand how to work on them. They do have drawbacks: because they use air flow to perform their task, they are sensitive to air density changes due to altitude, losing power at higher altitudes. They also are sensitive to dirt, long periods of storage, and orientation, i.e. if your bike tips over, fuel will spill out on the ground.

Electronic Fuel Injection is the modern solution to the task of mixing fuel and air. It utilizes a small computer and sensors to determine the instantaneous requirements of the engine, which enables better fuel economy and power. The fuel is pressurized by an electric fuel pump and fed to a fuel injector, which is a tiny little valve with a spray nozzle that injects a mist of fuel into the intake tract of the engine. Varying levels of power are achieved by how long the injector is held open. Because air and atmospheric pressure are not used to deliver fuel, and because their sensors can instruct the computer to adapt to differences in air density, EFI systems are not affected by altitude. The downsides of EFI are that they are complex electronic systems which are not easily diagnosed and repaired if they quit working correctly. They are also more expensive to repair, but are extremely reliable.

Cooling Systems

Air cooled engines use fins on the cylinder to increase the surface area so the excess heat can be carried away by the wind rushing over them while the bike is moving. Air cooling is simplicity itself. There are no moving parts and no maintenance. However, it cannot shed much heat when the bike is sitting still, leading to overheating. Air cooling limits the amount of heat which can be removed from the engine, which creates an upper limit of how much power the engine can create. Air cooling efficiency is also linked to ambient air temperature – it works best in cool air – not as well in hotter air. Burning fuel inside the engine creates power. This creates extreme heat inside the motor. Most of this energy is used to generate power, but some of it generates heat. The excess energy which cannot be turned into power becomes heat, and must be removed from the engine, or the internal parts will be melted or damaged. Removing excess heat is called Cooling.

Liquid cooling is the modern solution to this task. Most bikes sold today are liquid cooled. The system requires a pump, water passages around hot areas of the engine, a liquid coolant, a radiator, a thermostat, and a fan. Heat is carried by the liquid coolant to the radiator, where it is dispersed into the ambient air (just like on air-cooled bikes, only much better!).

If the bike is stationary and the coolant temperature rises, the thermostat senses this and turns on the fan, which forces air over the radiator, shedding excess heat. While the system is more complicated, it is completely proven, reliable technology. With the additional components and liquid, the system weighs more. Developments in manufacturing technology have produced significant weight savings in other areas, so liquid cooled engines are much lighter than they used to be, and this makes them the superior choice from a technological standpoint. The only maintenance requirement is to keep the coolant full, and flush the system every several years.

Transmission

Transmissions are usually incorporated into the engine cases (called a unit trans) but some bikes have separate transmissions, driven by a belt or chain. Transmissions are necessary because they multiply the RPM of the engine using different gear ratios to suit a wide range of road speeds. Gear ratio changes are done by moving the Shift Lever. Older bikes usually have fewer “speeds” i.e. gear ratio combinations. Newer bikes have more speeds to allow for better acceleration. Transmissions usually have 4, 5 or 6 speeds.

Some manufacturers have produced automatic transmissions which electronically select gear ratios, but these are unusual in general.

Most scooters use a “Continuously Variable Transmission”, or “CVT” which keeps the motor near its peak power output during acceleration, getting the most performance out of modest power. CVT trans have not been offered on full-size motorcycles despite offering potentially superior performance. Generally, transmissions do not require maintenance unless they are separate from the engine cases, in which case oil changes are required.

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