The Evolution Of Dirt Bikes: Fuel Injection's Arrival

Fuel injection technology in dirt bikes has been around since the early 2000s, with Suzuki introducing the first fuel-injected motocross bike in 2008. Since then, leading manufacturers such as Yamaha, KTM, Honda, Kawasaki, and Husqvarna have all released dirt bike models with fuel injection systems, with Yamaha's fifth-generation YZ450F making waves as the first four-stroke dirt bike with fuel injection. The benefits of fuel injection include seamless engine startup, improved performance, and reduced maintenance, as fuel injection can power your bike smoothly through all conditions, regardless of temperature or altitude. However, the switch to fuel injection in dirt bikes also comes with a higher price tag, and some riders may prefer the customizability and repairability of carburetors.

Fuel injection vs carburettor

Fuel injection and carburettors are both fuelling systems used in motorcycles, with fuel injection now being the standard in newer models.

Carburettors are a simple and cost-effective way of delivering fuel to the engine. They are a mechanical component that mixes the right amount of air and fuel before sending it to the engine's cylinders. Carburettors use a tube to feed an air-fuel mixture into the cylinder, with an air filter attached to one end. The velocity of the air passing through the tube is increased by restricting the air passage area in the middle of the tube, creating an area of low pressure. This then draws fuel from a jet near the venturi through suction. The amount of air sucked into the carburettor is controlled by a valve connected to the accelerator grip, which controls the air-fuel flow.

Fuel injection systems, on the other hand, are a more complex and accurate way of delivering fuel to the engine. They use electronic sensors and computerized control units to monitor and adjust the fuel injection process, ensuring the optimal air-fuel ratio for combustion. The fuel injection nozzle goes directly inside the combustion chamber, and the pressurised fuel is atomised as a homogeneous mist, allowing for efficient and clean combustion.

Advantages of carburettors

Carburettors are cost-effective, simple in operation, and easy to repair or replace. They allow users to tune them to their specific requirements and can be serviced or replaced without touching the engine as they are not integrated into it.

Disadvantages of carburettors

Carburettors are not the most efficient systems and have a dated design. They have a slight lag, resulting in a relatively slow throttle response, and certain components are delicate and prone to damage. The air-fuel mixture also fluctuates, affecting engine smoothness.

Advantages of fuel injection

Fuel injection systems optimise the air-fuel mixture, allowing for cleaner and more efficient combustion, sharper throttle response, better fuel efficiency, and marginally more power. They are typically maintenance-free and do not require complex tools for repairs.

Disadvantages of fuel injection

Fuel injection systems are substantially more expensive than carburettors and cannot be repaired or customised without expensive tools or ECU maps.

While fuel injection is becoming the standard in motorcycles, there is still a place for carburettors, especially in dirt bikes. For dirt bike riders who are not mechanically inclined, carburettors may be preferred as they are generally easier to maintain and repair. Additionally, for riders who plan on riding at various elevations, fuel injection systems can automatically adjust the air-fuel ratio accordingly, whereas carburettors would need to be manually rejetted for optimal performance. However, some riders prefer the "personality" and "spunky" powerband of carburetted dirt bikes over the linear power delivery of fuel-injected bikes.

Suzuki RM-Z 450

The Suzuki RM-Z 450 is a motocross bike that was first introduced with electronic fuel injection in 2008. This was a significant development as it was the first production motocross machine to feature this technology. The fuel injection system in the RM-Z 450 offers better roll-on power and improved low-to-mid-range delivery.

The RM-Z 450's fuel injection system consists of an internal battery-less fuel pump located inside the fuel tank and a specially designed Keihin carburettor. The fuel pump is located within the fuel tank, while the fuel injector is positioned to spray fuel directly at the butterfly valve, improving fuel/air atomization. This design also eliminates complex control linkage, allowing the rider to feel a more direct connection to the engine.

The RM-Z 450's 449cc, liquid-cooled, four-stroke engine is known for its high peak horsepower and strong torque at lower engine speeds, resulting in excellent throttle response throughout the entire rev range. The engine's compact aluminum cylinder is coated with Suzuki Composite Electrochemical Material (SCEM) for durability, lightweight, and efficient heat transfer. The piston casting includes strengthening ribs near the wrist pin bosses to match the engine's high horsepower.

The RM-Z 450 also features Suzuki's MX-Tuner 2.0, which allows riders to adjust the fuel injection and ignition system settings using a smartphone application. This provides quick and easy EFI tuning, with the ability to pre-program settings for different tracks or weather conditions. Additionally, Suzuki's proven fuel couplers are included for simple fuel adjustments to suit riding conditions.

The overall design of the RM-Z 450 prioritizes nimble handling and cornering performance. The twin-spar aluminum alloy frame and twin-beam swingarm balance strength and weight, with the frame being 700 grams lighter than the previous generation. The hexagonal aluminum rails on the sub-frame contribute to a slimmer appearance and easier air filter service. The Showa coil spring front fork and BFRC (Balance Free Rear Cushion) shock absorber further enhance the bike's handling and performance.

Yamaha YZ450F

The Yamaha YZ450F is a four-stroke racing motocross bike built by Yamaha Motor Corporation. It is credited with starting the four-stroke dirt bike revolution. The YZ450F was introduced in 2003 as the successor to the YZ426F.

The first generation of the YZ450F lasted from 2003 to 2005. The engine displacement was increased to 449 cc, and the weight was decreased to 233 pounds. The bike produced over 52 horsepower, but many thought it had too much power for a motocross track.

The second generation of the YZ450F lasted from 2006 to 2009. This generation saw a major update in 2006, with over 300 parts changed and improved. The power was smoothed out, and the bike was made easier to ride, addressing the criticism that the previous generation had too much power.

The third generation of the YZ450F lasted from 2010 to 2013. On September 8, 2009, Yamaha introduced its all-new YZ450F with fuel injection. The engine cylinder is rearward-slanted, the crank rod angle has been changed, and the 5-valve head is now a 4-valve head. The entire package is designed to create a mass-centralized bike.

The fourth generation of the YZ450F lasted from 2014 to 2017. The 2014 YZ450F retained the backwards-slant engine and featured a wet-sump engine and an easy-access air filter.

The fifth generation of the YZ450F began in 2018 and continues to the present. The 2018 YZ450F features electric start and a Mikuni fuel injection system. The kickstarter was removed for this generation.

KTM 250 EXC TPI

The KTM 250 EXC TPI is an extreme machine and a technological showcase for the brand. It features groundbreaking TPI (Transfer Port Injection) technology, which is a type of fuel injection system. This system offers several advantages over traditional carburettor engines, such as reduced fuel consumption, lower emissions, and the elimination of the need for fuel premixing and manual carburation adjustments.

The TPI system on the KTM 250 EXC consists of injectors positioned on both sides of the cylinder, delivering fuel to the rear transfers. The downward injection design, combined with rising air, results in excellent atomization, leading to more efficient combustion, fuel savings, and reduced emissions. The engine management system (EMS) calculates ignition timing and fuel injection quantities based on various sensor readings, including intake air pressure, throttle position, water temperature, and ambient air pressure. This ensures optimal performance regardless of altitude and temperature fluctuations.

The 2020 version of the KTM 250 EXC TPI introduced state-of-the-art exhaust valve control, a sophisticated mechanism for the side exhaust outlets, and a new tailpipe configuration, all contributing to enhanced engine performance. The new models also allow the use of conventional spark plugs. The engine design includes a side-mounted balancer shaft that minimises vibrations, making the riding experience more comfortable and less tiring.

The KTM 250 EXC TPI's lightweight chromoly steel frame has been designed with feedback from factory riders, providing excellent rider feedback. The engine has been rotated forward by one degree, optimising front-wheel grip and improving cornering. The aluminium monoblock swingarm offers exceptional strength while minimising weight. The suspension system, consisting of the WP XPLOR USD fork and PDS shock absorber, delivers a smooth and stable ride, even on rocky and choppy terrain.

The power delivery of the KTM 250 EXC TPI has been significantly improved. The addition of an ambient pressure sensor provides more information to the ECU, resulting in stronger and smoother power delivery, making it well-suited for both open terrain and technical conditions. The redesigned radiators, mounted 12mm lower, contribute to a lighter feel and improved agility.

Overall, the KTM 250 EXC TPI sets a benchmark for intuitive handling, unique style, and top performance, demonstrating KTM's commitment to 2-stroke development and innovation.

Husqvarna TE250i

The Husqvarna TE250i is the brand's two-stroke enduro model that features electronic fuel injection (EFI). The "i" in the model name indicates this. Husqvarna introduced the fuel-injected model in 2018, and it was the first of its kind.

The TE250i has a heavier ignition rotor than its motocross counterpart, the TC 250. This produces more inertia and improves control at lower rpm, which is valuable when traversing single-track trails where slow riding technique is key. The TE250i's engine performed well at super-low rpm in first gear but even better in second gear on more technical single-track trails.

The TE250i's fuel-injected engine produced 40.74 hp at 8,230 rpm and 27.49 pound-feet of torque at 7,580 rpm. Husqvarna updated the cylinder for better performance, added a new water pump casing to improve cooling, lightened the subframe, updated the suspension, and swapped out the exhaust for the 2020 model.

The TE250i's fuel injection system consists of a fuel tank, an electric pump, a hose, and an injector. The fuel pump is assembled at the bottom of the fuel tank and consists of a rotor, magnet, impeller, brushes, control valve, and relief valve. The ECU checks the ON/OFF condition of the pump.

The TE250i has six sensors that collect data for the Engine Management System (EMS), which controls the ignition, fuel injection, oil pump, and radiator fan. These sensors include a roll-over sensor, a crankcase pressure sensor, an engine-speed sensor, a throttle-position sensor, a coolant-temperature sensor, and an intake-air temperature sensor.

The TE250i also features a stronger generator and regulator, a throttle body made by Dell'Orto, and a separate 700cc oil tank mounted under the fuel tank. The oil tank is challenging to fill and gauge, but a warning light indicates when the oil is running low.

Overall, the Husqvarna TE250i is an impressive package and a critical development for the future of two-stroke motorcycles.

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