What is Straight Twin Engine?

In this post, we will going to learn what is straight twin engine and how it works. This article will also go over the working, advantages, and uses.

What is Straight Twin Engine?

A straight-twin engine consists of two piston engines arranged in a line along a single crankshaft. It is sometimes referred to as an inline-twin, vertical-twin, or parallel-twin.

Inline twin engines and parallel twin-cylinder engines are two common layouts for twin-cylinder engines, which have two pistons and cylinders in a single engine block.

The location of the cylinders, the crankshaft angle, and the piston movement are the main differences between them. Straight-twin engines have employed a variety of crankshaft layouts; the most popular ones are 360°, 180°, and 270°.

Straight-twin engines are most commonly found in motorcycles. Although, they are also used in automobiles, maritime boats, snowmobiles, jet skis, all-terrain vehicles, tractors, and ultralight aircraft.

Terms of Straight Twin Engine

The straight-twin configuration is also known as “parallel-twin,” “vertical-twin,” or “inline-twin.” A number of these phrases are frequently used together, but they originally had different meanings related to the crankshaft angle or engine orientation.

Parallel Twin

“Parallel-twin” is a term used in the UK for engines with a crankshaft angle of 360 degrees, since both pistons are acting in the same direction.

Vertical Twin

The term “vertical-twin” was used to describe engines that have a 180-degree crankshaft angle, which causes the pistons to move in opposing directions.

Inline-Twin

Usually, the terms “straight-twin” and “inline-twin” were used to describe any crankshaft angle. “Inline-twin” is sometimes used for motorcycles to refer to either a longitudinal engine orientation (the crankshaft aligned with the chassis) or a U-engine (the crankshafts are actually transverse).

Designs of Straight Twin Engine

Straight twin engines are more affordable to construct, have a simpler design, and are more compact than V-twin and flat-twin engines. There may be vibration in straight-twin engines because of the irregular firing interval in 180° crank engines or the large uncounted reciprocating mass in 360° crank engines.

Additionally, inline-twins have greater vibration and torsional torque reactions.

#1 Crankshaft Angle

For straight-twin engines, the most typical crankshaft layouts are 360°, 180°, and 270°.

360° Degree

In a 360-degree crankshaft engine, both pistons rise and fall simultaneously. The firing time between cylinders is offset, with one cylinder firing during the first crankshaft rotation and the other firing during the next rotation.

Unlike other inline twin engine crank arrangements, this setup produces an even 360-degree firing interval. The 360° engines have the ability to use a wasteful spark system with a single ignition system for both cylinders.

During the 1930s, most British four-stroke straight-twin motorcycle engines used a 360 degree crankshaft to eliminate uneven intake pulsing, thereby reducing the need for twin carburetors.

Although most Japanese motorcycles with 250–500 cc engines adopted 180 degree crankshafts in the 1960s, several smaller and larger engines stuck with 360 degree crankshafts.

180° Degree

A 180 degree crankshaft engine has one rising piston and one falling. The firing interval in a four-stroke engine is uneven, with the second cylinder firing 180 degrees after the first, then 540 degrees later, when the first cylinder fires again.

The irregular firing interval creates vibrations, resulting in a ‘lumpy’ power output. A 180° engine requires an individual ignition mechanism for each cylinder.

The 180° twin engine experiences reduced pumping losses in comparison to a 360° twin engine because the crankcase displacement remains generally constant during piston movement.

During the 1960s, 180-degree crankshafts become popular by Japanese motorcycle manufacturers. It is because of their ability to produce larger power outputs at higher rpm due to their increased smoothness.

A 180 degree crankshaft is commonly used in two-stroke engines because it produces two equally spaced power strokes each revolution.

270° Degree

A 270-degree crankshaft engine has one piston following three quarters of a rotation behind the other. This causes an irregular firing interval, with the first cylinder firing 270 degrees after the second, and the second cylinder shooting 450 degrees after the first.

This is the same pattern as an engine with a 90 degree V-twin, and as a result, the exhaust from both configurations sounds similar to a “pulsing” sound.

270 degree straight-twin engines don’t have both pistons stationary at the same time (as in 90 degree V-twin engines), so the net momentum exchange between the crank and pistons is reduced.

The 1996 Yamaha TRX850 and Yamaha TDM featured the first 270-degree straight-twin motorcycle engines in production.

#2 Main Bearings

Compare to V-twin engines, which have the ability to utilize a single crank pin for both connecting rods, straight-twin engines have individual crank pins for each cylinder.

There are usually two main bearings in vintage British straight-twin motorcycle engines (such as Triumph, BSA, Norton, and Royal Enfield). In the late 1950s, most Honda straight-twin engines used four main bearings.

Main bearings on straight-twin engines from later models were four or sometimes three.

Advantages of Straight Twin Engine

The following are the main advantages of straight twin engine:

Refinement

Compared to single cylinder engines, twin cylinder engines are more refined due to their superior balance. Twin cylinder engines can rev higher and be less vibrational because the movement of the two pistons better counterbalances the internal stresses within the engine.

More Power

Twins are more enthusiastic and intended than their single cylinder since they have less weight and a lower cubic capacity per cylinder to displace. Typically, twin cylinder engines have a higher specific output. Also, these engines spin more quickly, giving the rider a sportier sensation.

Sportier Sound

A single cylinder engine has a “bassier” beat that some users may enjoy, but not everyone finds it attractive. Bike fans frequently seek for the more harmonic sound that twin cylinder engines generate.

Disadvantages of Straight Twin Engine

The following are the disadvantages of straight twin engine:

Expensive

Twin cylinder engines have higher production costs because of its larger size, more components, and more complex design.

Bigger and Heavier Overall

While twin engines have fewer internal components compared to single engines, the former has twice as many. Twin-cylinder engines are heavier due to their bigger engine area and more parts, which also has an adverse effect on the bike’s overall weight.

Higher Cost of Maintenance

Twin cylinder engines are not only expensive to buy, but their complex design also raises the expense of replacing their parts. Also, twin-cylinder engine repair is more costly.

Less Efficiency

The fuel consumption of twin cylinder engines is generally higher than that of single cylinder engines.

Applications of Straight Twin Engine

The Hildebrand & Wolfmüller, which debuted in 1894, was the world’s first motorcycle produced with a straight twin engine. The second motorcycle model put into production was the Werner Motocyclette, which had vertical cylinder straight-twin engines in 1903.

Straight-twin engines became more widely used by other brands after the 1938 Triumph Speed Twin became a successful motorcycle. During the 2000s, straight-twin engines have been produced by a number of Japanese companies, including BMW, primarily for middleweight vehicles.

Several automobile engines have used straight-twin layouts over the years, even though two-cylinder engines are quite rare. An 1895 version of the Phoenix engine from Daimler Motors was the first known straight-twin engine. These engines were installed in Panhard vehicles that year.

In 1898, the Decauville Voiturelle was another car with a straight twin engine. Straight-twin engines have been utilized in many small vehicles, including the Fiat 500, Subaru 360, NSU Prinz, Mitsubishi Minica, Honda N360, Honda Z600, and the Tata Nano.

Straight-twin motors are frequently employed as jet pump motors, outboard motors, and inboard motors. Other potential uses include all-terrain vehicles, snowmobiles, personal watercraft, and tractors (such as several John Deere models up until 1960).

Wrapping It Up

That’s it. Thanks for reading. I hope I have covered everything. It would be helpful if you could let me know if there was anything I missed or if you have any doubts about anything I wrote.

FAQs

How do twin engines work?

A twin-engine design is sometimes known as an inline twin, vertical twin, or parallel twin. It features two cylinders positioned in a line along the crankshaft. When an engine has a 360-degree crankshaft, both pistons rise and fall simultaneously. One cylinder fires during the first crankshaft spin, while the other cylinder fires during the next rotation, causing an offset in the firing time between the cylinders.

Which is better inline twin or parallel twin?

An inline twin-cylinder engine is an effective choice if you desire a smooth, comfortable, and economical ride. A parallel twin-cylinder engine, on the other hand, offers a more potent, thrilling, and responsive ride.

Why are V-twin engines good?

They have improved overall linear performance and generates a unique V-twin engine sound. To help lower the total weight, it makes use of a common crankshaft and crankpin. It produces twice as much power and torque as a single-cylinder engine.

Which bikes have two cylinders?

The following are some of the future twin-cylinder motorcycles: Kawasaki X-Versys 300, Royal Enfield Classic 650, Royal Enfield Scrambler 650, Royal Enfield Shotgun 650, and Royal Enfield Bullet 650.