Automobiles Engine Cylinder Bore stroke Ratio

 Bore stroke Ratio

In the automotive/automobile engines are a large variety of classifications of the engine one of the basic of engine cylinder layouts or bore stroke ratio. The Bore and stroke ratio directly affects the vehicle engines overall performance, engine characteristics, application, engine life, and cost aspects in the two-wheeler and four-wheeler vehicles. Bore and stroke are major key roles played in the engine characteristics and engine structure. It is a simple word to say that to understand engine cylinder bore and stroke Ratio equal to bore/stroke. 

Ratio = Bore/Stroke.

Automobile engines are categorised into three sections according to bore stroke ratio. These are: 

• Square Engine 

• Over-Square Engine (Shorter Stroke Engine)

• Under-Square Engine (Longer Stroke Engine)

Square Engine 

Square Engine have to equal bore to stroke ratio hence the ratio is 1 one. That means when you look square engine with the same sizes as the engine cylinder bore and stroke. Bore= Stroke (Engine Cylinder Bore equal to the length of the engine stroke).

 Let's take example Yamaha model R15 motorcycle is a square engine cylinder bore 58 mm and stroke is 58.7 mm. FZ Fi Yamaha has 57.3 bores and 57.9 mm length of stroke, similarly, Honda Unicron 150 model has a cylinder bore 57.3 and stroke 57.8 mm. The slight length of the stroke is large compared to the cylinder bore but come on the same size as the round finger. These types of engines are the most commonly used two-wheeler vehicles. Four Wheeler vehicles do not prefer square engines, some rare square engines use in the four-wheeler vehicle. Heavy-duty and light-duty motor vehicles are commonly used under square/Longer stroke diesel engines vehicles. 

Over-Square Engine 

Over Square engine also called shorter stroke Engine. Over square engine has a larger bore and shorter length of the cylinder stroke. So the ratio is more than one. Bore > Length of Stroke (Bore larger than Stroke).

This engine has larger engine valves and also multi-valves that can be used. It higher rpm (revolution per minute) is developed thus higher power and more mixture intake due to the multi valves are used. So that provides more horsepower. These types of engines used high-speed sports bikes and high-performance cars and maximum seen on Formula1 and sports cars. Over square engine has high fuel consumption and heat transfer increase due to more surface area and hence efficiency decrease. It also more emission increase and quiet noisy engine.

Some two-wheeler is in the Indian markets KTM motorcycle 200 Duke model engine cylinder bore is 89 mm and stroke is  60 mm similarly Kawasaki Ninja engine cylinder bore is 62 mm and length of engine stroke is 44 mm and latest Bajaj Pulsar 150 CC motorcycle engine has to Bore

 60.7 mm and stroke are 56.5mm. Bajaj Pulsar made by shortening the Engine Cylinder Stroke and other sports bikes uses an over-Square engine mainly focusing on top speed/acceleration and high-performance motorcycles and bikes.

Under-Square Engine 

Under-Square engines are most widely used in both two-wheelers and four-wheeler vehicles. Under square engine has born to stroke ratio less than one. That Means Cylinder Bore less (narrow Bore) than the length of the engine cylinder stroke. Bore < Length of Stroke.

This engine is also called Longer Stroke Engine. The engine has a friction increase due to the longer length of the engine cylinder stroke (which means longer travels engine piston). It uses tiny engine valves due to a narrow cylinder bore, but maximum torque is achieved at lower rpm. as compared to Over-Square Engine. These engines crankshaft stress increases and engine efficiency increases perhaps less heat transfer as fewer surface area.

Under-Square (Longer Stroke) Engines uses heavy-duty diesel engines like tractors, excavators, trucks buses and commercial vehicles and even in uses for ships engines. These engines are torquey and fuel economy engines.  In the Indian market two-wheeler engines most the majority average and fuel economy motorcycles and bikes which mean below 150 Engine CC, motorcycles and bikes used under Square Engine but the manufacturer has their engines are well-tuned and refined for the customer need. Have an option for looking for the customer different segment motorcycle and bikes in the two-wheeler markets.

Let's take fewer motorcycle examples-

Royal Enfield Classic 350 model. Engine Cylinder Bore has 70 mm and length of stroke 90mm. New J-series Engine 350 Royal Enfield motorcycle Cylinder Bore is 72mm and 85.5mm Stroke. Honda H'ness CB 350 model motorcycle 70mm bore and 90.5 Stroke. Most cruiser motorcycles and bikes like Harley Davidson, Benelli Imperiale 400, Jawa 42 etc. These all simple or normal motorcycles and heavy-duty commercial diesel engines prefer to be made on under- square engines platform. They mainly force fuel economy and higher torquey performance.