How Do Rotax Engines Work?

by Richard Rowe
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In many ways, Austrian manufacturer Rotax's signature two-stroke engines are just like any other two-stroke out there; they work in the same basic way, have all the same basic parts and do all the same things. But Rotax engines have long been known for their enormous power output relative to weight and displacement. Like most things Austrian and German, the secret is all in its engineering and attention to detail.

Two-Stroke Basics

A two-stroke engine works by using the piston to cover and uncover intake and exhaust ports in the cylinder walls, instead of relying on a camshaft and valvetrain like a four-stroke engine. Starting at the top of th piston's stroke, an explosion of air and fuel shoves the piston down. As the piston goes down, it uncovers the exhaust port, which releases exhaust gases from the cylinder. Next, the piston uncovers the intake port -- opposite side of the cylinder from the exhaust -- and allows fresh air and fuel to enter the cylinder. The inertia of escaping exhaust gas sucks air and fuel through the intake port. Inertia drives the piston back up, covering both ports and compressing the air-fuel mixture against the cylinder head. The spark plug ignites that mixture and the cycle begins again.

The Basic Approach

Rotax engines are known for power an reliability, but the approach Rotax uses seems a bit antithetical to the one used by most two-stroke manufacturers. Horsepower is a function of torque and engine rpm; if you have more rpm, you need less torque, and vice versa. So, the simplest way to increase horsepower is to run the engine at higher rpm. In the karting world, Yamaha KT100S models typically rev to about 16,000 rpm and Leopard engines to about 17,000 rpm. The equivalent Rotax BRP makes the same or more horsepower revving to a comparatively tame 13,000 rpm. That might seem like a bad thing, but the end result is less engine wear and a flatter, more useable torque curve.

The Power Valve

In many ways, the Rotax engine's power valve is the secret to its success. The power valve works something like a VTEC system in a four-stroke engine, altering intake and exhaust valve timing according to rpm. But a two-stroke engine doesn't use valves like a four-stroke engine does; its intake and exhaust valve timing is dictated by the height and position of the intake and exhaust ports. So the only way to alter timing is to physically change the size of the ports. The Rotax's power valve is essentially a plate that drops down from the top of the exhaust port opening. With the PV in the closed position, the port roof is lower and the port is smaller, which works better at low rpm. At about 7,500 rpm, the PV rises, lifting the port roof to boost top-end horsepower.

Full-Roller Assemblies

Rotax is one of the few two-stroke manufacturers in the world to religiously use roller bearings for every rotating component on the engine. The ends of the crankshaft, the crankshaft-to-rod pin and the piston pin all use precision-engineered roller bearings instead of the conventional flat bearing. Roller bearing assemblies are more expensive, more complicated and more difficult to engineer, but they also offer far less resistance to movement, increase engine longevity and lower temperatures than flat bearings. At 10,000-plus rpm, roller bearing assemblies can make a huge difference in power and engine wear, and they're part of what allows the Rotax engine to make power at lower rpm than equivalent engines.

Tuned Exhausts and Timing

When exhaust gases escaping the two-stroke's cylinder pull air-fuel through the cylinder, they almost inevitably pull a certain amount all the way through the cylinder and out through the exhaust port. A "tuned exhaust" uses an expansion chamber, which looks like two cones placed base-to-base, to catch pressure waves coming out of the engine and bounce them back toward the motor. At a certain rpm, these pressure waves will coincide with the air-fuel charge coming through the exhaust, shoving it back into the engine and "supercharging" the motor. Again, Rotax isn't the only manufacturer to use tuned pipes, but it is one of the few manufacturers to use completely fixed pipes that don't allow for user adjustment. This is so the end-user can't play with exhaust tuning, which would mess up power valve and electronic ignition timing.

Features and Issues

Rotax engines also use a reed valve in the intake, just after the carburetor. The reed valve is a triangle-shaped screen that acts as a one-way valve, keeping air pressure in the engine from working its way backward through the carburetor. Rotax engines also typically use breakerless, capacitive discharge, fully-electronic ignitions sourced from motorcycle builder Ducati. Rotax engines don't have too many problems compared with others, but the power valve does create one distinctive quirk in the driving experience. Rotax racing engines, those used in karts and snowmobiles, often hesitate a bit after exiting a corner. This hesitation mimics a carburetor or ignition problem, but is actually a result of the PV quickly closing and opening. Rotax drivers have to adopt a slightly different driving style than operators of other engines, smoothly feeding in the throttle rather than just flooring it upon corner exit.

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