Pair of Aces - KTM’s all-new two-stroke enduros pack a real punch
By Team TMX on 18th May 17
KTMs pair of 2018 fuel-injected two-stroke enduro models finally broke cover at a global launch held in Erzberg, Austria recently.
Now coming with TPI (Transfer Port Injection) technology as standard, the 250 and 300cc EXC machines look set to blaze a trail in terms of fuel economy, performance and usability as the new motors run remarkably clean while offering consistent performance without the need for constant re-jetting.
The TPI system is managed by a newly developed Electronic Control Unit (ECU) that sits underneath the seat.
By using various sensors that measure ambient and intake air pressure, throttle position, engine speed and ambient and coolant temperature the ECU is able to make automatic changes to ignition timing and how much fuel is injected.
It's almost like having a team of mechanics chasing you round with a box of brass jets making thousands of changes per second.
Further labour savings are made as the question over whether the bikes will require pre-mixed fuel or not is finally answered with ‘no' being the answer.
A tank-fed, electronically-controlled oil pump takes care of lubrication meaning neat fuel can go in the tank.
While a throttle body sits where the carburettor would normally go the injectors themselves fire fuel directly into the transfer ports.
Like any other two-stroke, engine air is drawn into the lower end when the piston goes up and is forced up through the transfer ports when it drops back down.
With the fuel injected directly into the transfer port the upward force will push the fuel infused mist into the combustion chamber where compression and a spark will produce power. A regular style power-valve helps manage power output.
With less oil being burnt, KTM claim exhaust smoke will be much less – around 50 per cent – with the 0.7 litre oil tank delivering the optimum fuel/oil mix for just over five tanks of fuel – that's 45 litres at nine litres per thankful.
While it's the motor and injection system that's the big news other modifications have been made to the 2018 models that also benefit from e-starters powered by super-lightweight lithium ion batteries, Brembo brakes, self-adjusting hydraulic clutches, KTM ‘No Dirt' footpegs and gear shift levers, KTM toolless air filter boxes, quick-release fuel lines, polished hubs, WP Xplor 48 split forks and WP Xplor PDS shock absorbers.
Throttle Body
The TPI engines are fitted with a newly-developed 39mm throttle body made by Dell'Orto.
Power is controlled by a throttle valve and two throttle cables via a newly developed handlebar throttle assembly group.
Intake air data is provided to the engine management system by a Throttle Position Sensor (TPS) while an adjusting screw allows the idle speed to be set.
A cold start device is also provided. Oil supplied by the automatic oil pump via an oil suction pipe is mixed with the incoming air to lubricate the moving engine parts.
Euro 4 compliant
The European stage 4 emission standards for motorcycles have been in effect since January 1, 2016, using the World Motorcycle Test Cycle (WMTC) and once again establishing stricter limits for motorcycle exhaust emissions.
Thanks to the highly efficient combustion control of the TPI system and precise automatic engine lubrication, the new KTM EXC models will be Euro-4-compliant, not the least due to their significant reduction of hydrocarbon emissions.
Oil Pump
An oil pump mounted underneath the fuel tank delivers the lubricant to the throttle body, where tiny quantities are mixed with the incoming air in order to lubricate the moving engine parts.
Using engine speed and TPS data, the engine management system's precise electronic control of the oil pump ensures the perfect amount of oil for any given conditions, loads and engine speeds is added.
The filler cap assembly for the oil tank is located between the fuel tank and steering head, utilising a hose in the upper frame tube to conduct the oil to the storage container.
Automatic, electronically controlled oil injection reduces exhaust smoke by up to 50 per cent over conventional pre-mix lubrication, with an average fuel-to-oil ratio of 1:80.
Cylinder
While the core dimensions and layout of the TPI cylinders are carried over from the carburettor model with bore diameters of 66.4 and 72.0mm, respectively, the new cylinders feature two lateral domes holding the fuel injectors that inject fuel into the rear transfer ports.
This downstream injection guarantees excellent atomisation of the fuel with the upstreaming air for a highly efficient combustion and significantly reduced losses of unburned fuel.
The engine management system's intake pressure sensor is connected via a small-diameter tube in the back of the cylinder.
For a smooth and well-controlled power delivery, the sophisticated exhaust control of the carburetted EXC models is also retained.
Balancer Shaft
For optimal mass centralisation, the lightweight, die-cast crankcases hold the clutch shaft and the crank- shaft in a high position close to the centre of gravity supporting the handling of the bike.
In addition, this design allows the implementation of a lateral balancer shaft to minimise vibrations, enhancing comfort and reducing rider fatigue during long and demanding Enduro events and rallies.
Clutch
The KTM EXC TPI models employ the KTM-developed DDS clutch (damped diaphragm steel) with a highly resilient steel basket and extremely heat resistant clutch plates, a design distinguished by a considerably easier clutch operation than with conventional coil springs.
The diaphragm spring also leaves sufficient space for a damping system integrated into the clutch hub, which benefits both traction and durability. The hydraulic Brembo clutch mechanism is synonymous with a light action and highly controlled clutch modulation.
Transmission
The rugged six-speed transmission features widely spaced gear ratios optimised for enduro duty. It provides precise and easy shifting. The KTM ‘No Dirt' gear shift lever design prevents residue from blocking the joint of the gear shift lever for safe and reliable shifting in any conditions.
E-starter and battery
The EXC TPI models are equipped with e-starters located in a well-protected position on the undersides of the engines. A powerful but lightweight lithium ion battery provides convenient and reliable starting.
Subframe
The rear subframe made of lightweight aluminium sections is connected to the frame with four bolts and weighs a mere 900g – less than a bag of sugar.
Frame
The frames of all KTM off-road models are manufactured from lightweight sections of top-quality chrome-molybdenum steel tubing of various cross sections.
This type of frame design provides high torsional rigidity for best handling and rideability, while a certain longitudinal flexibility can absorb the wheel impact energy, supporting the suspension for reduced rider fatigue.
At the same time, the frame triangle serves to deliver the coolant from the cylinder head directly to the radiators.
In the TPI models, the modified frame design also serves to route engine oil from an added filler cap assembly, located between the fuel tank and the steering head, via a hose inside the upper frame tube to the oil tank.
Air filter and air box
The air filter box design offers maximum protection of the air filter against soiling while ensuring maximum air flow.
It uses a large Twin Air filter element mounted on a rigid cage which at the same time forms the air filter box bracket. The straightforward design also minimises the risk of installing the air filter or cage incorrectly.An original staple of practical KTM design, the filter is quickly and easily changed without tools in seconds. In addition, the new TPI two-stroke models are fitted with a new velocity stack to match the throttle body while at the same time accommodating an intake air temperature sensor.
Fuel tank and pump
All of KTM's enduro models feature lightweight, translucent fuel tanks that make it easy to gauge the fuel level from the outside. Along with the new injection system, the TPI models receive a new, nine litre tank with an integrated fuel pump system along with an additional fuel level sensor.
