How Stinky is Your Two-Stroke?

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ECUs for 2-stroke engines?  Here they come.

By now, most  engineers are comfortable with the notion that the old Benz carburetor and Kettering ignition have been relegated to the automotive history books, replaced by engine control units (ECU) that regulate fuel injection and spark timing according to algorithms that far more effectively achieve fuel efficiency and emissions control.  But those have been limited to 4-stroke engines and cylinder counts of four and above.  Last week, Freescale Semiconductor announced availability of ECUs for one- and two-cylinder 2-strokes.  That was a little shocking. 

Okay, the old 4-cycle car carburetors managed and distributors managed to be both elegant and kludgy at the same time – needle valves and butterflies to match inlet air flow to the proper air/gas stoichiometry, centrifugal force, balanced by vacuum to manage a mechanical boost converter – and a complex arrangement of cams and push rods to coordinate the dance of the inlet and exhaust valves. All with dozens of refinements that have always put me in awe of classmates who pursued ME degrees. But awesome as the engineering was, it was reassuring for an EE to find that it could be improved upon by bit-slingers.

On the other hand, I would have figured that 2-stroke engines were too simple to digitize.  Spark?  Put a magnet on the flywheel. Fuel? A simple, 2-needle carb and holes in the cylinder wall in lieu of valves.  Once you’ve set the jets, what else is there to do?

Well, as Majid Eshaghi, Freescale’s Powertrain Product Line Manager explained it to me, there are mandatory global efficiency requirements for 2-stroke engines on everything from small-displacement motorbikes, to outboard motors, lawnmowers and leaf blowers coming down the pike, and the companies that make the engines for these things need to get their acts together.  Clearly, the biggest market, and the one with the biggest need for lower emissions, is represented by those small-displacement two-wheelers that are the entry technology for family mobility in developing countries – or any place where average wages don’t leave much room for gasoline purchases after the basic necessities of life are taken care of.  Or any place where the sky is routinely brown.  In fact, Freescale introduced the products at a conference in Beijing.

What, exactly, has Freescale created?  In terms of function, the MC33813m, the single-cylinder ECU, consists of 5 integrated low-side drivers and two pre-drivers, and the dual-cylinder MC33814 has 6 integrated low-side drivers and three pre-drivers. Those low-side drivers are for the fuel injectors, the headlight, a fuel pump and a tach. The pre-drivers are for IGBTs or MOSFETs for switching the ignition coils (No more magnets on the flywheel.) and the thing that heats the O2 sensor.

Both devices  include a variable reluctance sensor (VRS) input circuit, a voltage pre-regulator that drives an external pass transistor and a separate five-volt internal regulators, one for Vcc and the other to drive the oxygen sensor.  There’s also a reset control and a diagnostic interface. The brains are external – Freescale MC9S12 MCUs.

Naturally, since Freescale is selling these to engine manufacturers, there’s a reference design, with software to track crank position, read analog sensors, read the gas gauge and ignition timing, and sync the ignition coils and fuel injectors, so even for mechanical engineers, the bit banging is relatively straightforward.

Samples are available now, if you want to play with some, but the pricing info Freescale gave the media ($1.69 and $2.06) is for 10-k volumes, so you’ll need to talk to Freescale.  (And besides, you’ll need the reference designs.)

The only downside I can see to all this is what it does relative to reliability and repair.  If the engine or bike maker does a poor job on assembling a regular “2-smoke,” any shade-tree mechanic can fix it.  With these new bikes, it’s going to take a while before there are enough wrecks to cannibalize, and the parts being swapped will be at the board level.  Still if it helps clear up the skies over Beijing, or prevents some other cities from going down that smoggy path, it’s a good thing.

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Don Tuite

Don Tuite covers Analog and Power issues for Electronic Design’s magazine and website. He has a BSEE and an M.S in Technical Communication, and has worked for companies in aerospace,...
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