If you’ve ever ridden a city bus, you may have wondered about the massive engine that propels these large vehicles down the road. Bus engines come in different sizes and configurations, but most modern buses are equipped with powerful diesel engines containing multiple cylinders.

If you’re short on time, here’s a quick answer to your question: most full-size transit buses have 6-cylinder diesel engines, while larger coaches tend to have 8 or more cylinders.

In this approximately 3000 word guide, we’ll take an in-depth look at bus engine cylinders, exploring the different engine types used, the factors that determine cylinder count, and the pros and cons of different cylinder configurations.

Transit Bus Engine Types

When it comes to transit buses, there are several different types of engines that can be found under the hood. Each type of engine has its own unique characteristics and advantages. In this article, we will explore three common types of transit bus engines: diesel engines, CNG & LNG engines, and hybrid drivetrains.

Diesel Engines

Diesel engines have been a popular choice for transit buses for many years. These engines are known for their durability, reliability, and fuel efficiency. They are powered by diesel fuel, which is ignited by compression rather than a spark.

Diesel engines typically have a higher torque output compared to gasoline engines, making them ideal for heavy-duty applications like transit buses.

According to a study conducted by the American Public Transportation Association (APTA), diesel engines power approximately 90% of transit buses in the United States. This is due to their proven track record and the availability of diesel fuel infrastructure.

CNG & LNG Engines

CNG (compressed natural gas) and LNG (liquefied natural gas) engines are another type of engine commonly used in transit buses. These engines utilize natural gas as a fuel source, which is considered to be more environmentally friendly compared to diesel fuel.

Natural gas engines produce lower emissions and have the potential for reduced operating costs.

CNG engines store natural gas in compressed form, while LNG engines store it in a liquid state. Both types of engines require specialized fueling infrastructure, which can be a barrier to widespread adoption.

However, many cities have invested in CNG and LNG fueling stations to support their transit bus fleets.

Hybrid Drivetrains

Hybrid drivetrains combine the use of an internal combustion engine (typically diesel or gasoline) with an electric motor. These drivetrains offer the benefits of both fuel efficiency and reduced emissions.

The electric motor assists the engine during acceleration and can also recover energy during braking, which is stored in a battery for later use.

Hybrid buses can achieve significant fuel savings compared to traditional diesel or gasoline-powered buses. According to the Federal Transit Administration, hybrid buses have shown fuel economy improvements ranging from 20% to 50% depending on the drive cycle and hybrid system design.

It’s worth noting that the adoption of hybrid drivetrains in transit buses is still relatively low compared to diesel engines. However, as technology continues to improve and battery costs decrease, hybrid buses are expected to become more prevalent in the future.

What Determines Cylinder Count?

When it comes to bus engines, the number of cylinders is not arbitrary. Several factors determine the cylinder count, including vehicle size and capacity, torque and power requirements, and fuel efficiency.

Vehicle Size and Capacity

The size and capacity of a bus play a crucial role in determining the number of cylinders in its engine. Larger buses typically require more cylinders to generate the power needed to transport a higher number of passengers and handle the additional weight.

Smaller buses, on the other hand, may have fewer cylinders as they have lower power and capacity requirements.

Torque and Power Requirements

The torque and power requirements of a bus engine also influence the cylinder count. Buses that need to tackle steep inclines or carry heavy loads may require more cylinders to generate sufficient torque.

Similarly, buses that need to reach higher speeds quickly may require additional cylinders to produce the necessary power. The engine’s cylinder count is optimized to meet the specific torque and power demands of the bus.

Fuel Efficiency

Fuel efficiency is another crucial factor that affects the cylinder count in a bus engine. Increasing the number of cylinders can provide more power but may also result in higher fuel consumption. Bus manufacturers strive to strike a balance between power and fuel efficiency, considering factors such as engine displacement, compression ratio, and combustion efficiency.

By optimizing the cylinder count, bus engines can achieve the desired balance between performance and fuel economy.

It’s important to note that the cylinder count is just one aspect of a bus engine’s design. Other factors, such as engine configuration (inline, V-shaped, or flat), valve design, and fuel injection technology, also contribute to its overall performance and efficiency.

6-Cylinder Bus Engines

Bus engines come in various sizes and configurations, but one common type is the 6-cylinder engine. These engines are designed to provide a balance between power and efficiency, making them a popular choice for buses of different sizes and purposes.

Power and Efficiency

A 6-cylinder bus engine offers a good balance between power and fuel efficiency. The six cylinders provide enough power to propel the bus forward smoothly, even when carrying a full load of passengers.

The additional cylinders allow for better torque and acceleration, making it easier for the bus to navigate uphill or over long distances. At the same time, these engines are designed to be fuel-efficient, helping bus operators save on fuel costs and reduce emissions.

According to a report by Bus and Coach Buyer, 6-cylinder engines are commonly found in mid-sized buses, with engine capacities ranging from 6.0 to 7.6 liters. These engines typically produce between 200 and 400 horsepower, depending on the specific model and configuration.


Maintaining a 6-cylinder bus engine is crucial to ensure its longevity and optimal performance. Regular maintenance, including oil changes, filter replacements, and engine inspections, is necessary to keep the engine running smoothly.

It is also important to monitor the engine’s coolant levels and address any leaks or coolant-related issues promptly.

In addition to routine maintenance, it is important to follow the manufacturer’s recommendations for engine maintenance intervals and procedures. This can help prevent major breakdowns and expensive repairs down the line.

Regularly checking for any signs of wear or damage and addressing them promptly can also contribute to the longevity and reliability of the engine.


There are several well-known bus manufacturers that utilize 6-cylinder engines in their vehicles. One example is the Volvo B8RLE, which is equipped with a 6-cylinder engine and has been praised for its power, efficiency, and low emissions.

Another example is the Mercedes-Benz O 500 series, which features a 6-cylinder engine known for its reliability and performance.

These examples showcase the versatility and effectiveness of 6-cylinder bus engines in various transportation settings. Whether it’s for city buses, intercity coaches, or even school buses, the 6-cylinder configuration offers a reliable and efficient solution for powering these vehicles.

8+ Cylinder Bus Engines

When it comes to bus engines, it’s common to find models with 8 or more cylinders. These engines are designed for heavy-duty applications and are well-suited to handle the demands of carrying passengers and cargo. Let’s take a closer look at why more cylinders are needed in bus engines.

When are More Cylinders Needed?

More cylinders are needed in bus engines to provide the power and torque necessary to move the vehicle and its occupants. Buses are typically larger and heavier than regular cars, requiring more force to accelerate and maintain speed.

With additional cylinders, the engine can generate more power and torque, allowing the bus to handle steep inclines and challenging driving conditions.

Power and Torque

Having more cylinders in a bus engine translates to greater power and torque output. This means that buses with 8 or more cylinders can accelerate faster and have more pulling power, making them better equipped for stop-and-go city driving or climbing steep hills.

The increased power and torque also contribute to a smoother and more comfortable ride for passengers.

Fuel Economy

While it may seem counterintuitive, bus engines with more cylinders can actually achieve better fuel economy. This is because the additional cylinders allow the engine to operate at lower RPMs, which reduces the strain on the engine and improves efficiency.

Additionally, modern bus engines are equipped with advanced fuel injection systems and engine management technologies that further optimize fuel consumption.

Durability and Maintenance

Engines with 8 or more cylinders are known for their durability and longevity. The robust construction and design of these engines can withstand the demands of constant use and heavy loads. Moreover, the individual cylinders in these engines can be serviced and maintained independently, making maintenance and repairs more manageable and cost-effective.

Case Studies

Several bus manufacturers have opted for 8+ cylinder engines in their models to provide the necessary power and performance. For example, the popular Volvo B8RLE bus features an 8-cylinder engine, delivering exceptional performance and fuel efficiency.

Similarly, the Mercedes-Benz OC 500 RF bus is equipped with a 12-cylinder engine, ensuring smooth and reliable operation.

Engine Downsizing Trends

As the world becomes more conscious about the environmental impact of vehicles, engine downsizing has emerged as a popular trend in the automotive industry. This trend focuses on reducing the size and number of cylinders in engines to improve fuel efficiency and reduce emissions.

Let’s explore the factors driving this trend and some potential drawbacks to consider.

Emissions Regulations

Stringent emissions regulations imposed by governments worldwide have been a major driving force behind the engine downsizing trend. These regulations aim to limit the amount of harmful pollutants released into the atmosphere by vehicles.

By reducing the size and number of cylinders, automakers can achieve higher fuel efficiency and lower emissions. This not only helps them meet regulatory standards but also contributes to a cleaner and greener environment.

According to a study conducted by the International Council on Clean Transportation (ICCT), downsizing engines by one cylinder can reduce CO2 emissions by up to 7%. This reduction can have a significant impact on reducing greenhouse gas emissions and combating climate change.

Advancements in Engine Technology

Advancements in engine technology have played a crucial role in making engine downsizing a viable option for automakers. The development of turbocharging and direct injection systems has allowed smaller engines to deliver the same level of power and performance as larger ones.

These technologies enable engines to generate more power from a smaller displacement, leading to improved fuel efficiency and reduced emissions.

For instance, Volkswagen has successfully downsized their engines while maintaining performance through the use of turbocharging. Their TSI engines, which are smaller in size but still provide excellent power, have gained popularity among consumers.

Potential Drawbacks

While engine downsizing offers numerous benefits, there are some potential drawbacks to consider. One of the main concerns is the increased strain on smaller engines, which can lead to higher maintenance costs and reduced engine lifespan.

Additionally, smaller engines may not provide the same level of power and performance as larger ones, which could be a drawback for certain applications such as towing or heavy-duty use.

However, with advancements in technology and ongoing research, automakers are continuously improving the reliability and performance of downsized engines. It is important to weigh the benefits and drawbacks of engine downsizing based on individual needs and preferences.


In summary, most standard transit buses are equipped with 6-cylinder diesel engines, which offer a balance of power and fuel efficiency for their size. Larger coach buses may have 8, 10, 12 or even 16 cylinder engines to generate enough torque to move a heavy vehicle.

But advancements in technology are allowing engine downsizing even in buses, with similar power outputs from fewer cylinders.

Understanding the engine that drives your bus allows you to better appreciate this engineering feat that transports millions of passengers daily. Next time you board a bus, take a moment to consider the hardworking engine that makes your ride possible.

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