TWO-WHEEL CHAT­TER

Could KTM’S trans­fer port fuel-in­jec­tion tech­nol­ogy se­cure a fu­ture for emis­sion-com­pli­ant two-strokes?

Cycle World - - Contents - By Kevin Cameron Il­lus­tra­tions by Jim Hatch

Two-stroke lovers have dreamed of this day: when a ma­jor man­u­fac­turer ap­plies mod­ern tech­nolo­gies to pro­duce pow­er­ful two-stroke bikes once more—with le­gal low emis­sions.

Two-stroke road bikes pretty much dis­ap­peared in the US af­ter 1984 be­cause car­bu­reted two-strokes must scav­enge (re­fill, while push­ing ex­haust gas out) their cylin­ders with fuel-air mix­ture. Be­cause ex­haust and trans­fer (fresh charge) ports must be open si­mul­ta­ne­ously for roughly 120 de­grees, it was in­evitable that such en­gines would “short-cir­cuit” some fuel to the ex­haust, pro­duc­ing high emis­sions of un­burned hy­dro­car­bons and us­ing roughly 30 per­cent more fuel than a four-stroke en­gine of the same power.

Then in the later 1980s/ early ’90s came di­rect fuel in­jec­tion, promis­ing to cut two-stroke emis­sions and fuel con­sump­tion to fourstroke lev­els. By elim­i­nat­ing the car­bu­re­tor and in­ject­ing the fuel di­rectly into the com­bus­tion cham­ber af­ter the ex­haust port had closed, it was made im­pos­si­ble for raw fuel to en­ter the ex­haust. Be­cause a two-stroke’s ex­haust port typ­i­cally closes at 83 to 90 de­grees BTDC and ig­ni­tion oc­curs at or slightly af­ter 20 de­grees BTDC, that left roughly one-fifth of a rev­o­lu­tion for get­ting the fuel into the cylin­der and evap­o­rat­ing it into eas­ily ig­nited va­por. An in­jec­tor ca­pa­ble of those tasks was highly spe­cial­ized and there­fore ex­pen­sive. But DFI showed that clean­ing up two-stroke emis­sions and re­duc­ing fuel con­sump­tion could be ac­com­plished.

Bom­bardier then found an­other path to the same goal—trans­fer port in­jec­tion, or TPI. Trans­fer ducts move the fuel-air mix­ture from its pre-com­pres­sion in the crank­case, up into the cylin­der. If in­jec­tion into these ports is timed cor­rectly, three things hap­pen: 1) no fuel reaches the ex­haust port be­fore it closes, 2) more time is made avail­able for get­ting the fuel into the en­gine, mak­ing ex­pen­sive DFI in­jec­tors un­nec­es­sary, and 3) air ve­loc­ity up through the trans­fer ducts is of­ten close to sonic, ideal con­di­tions for break-up and evap­o­ra­tion of fuel droplets are cre­ated.

KTM’S TPI aims two in­jec­tors against the flow in the trans­fer port pair far­thest from the ex­haust port. Its 66.4 x 72.0mm 250 sin­gle makes a claimed 49 hp at 8,500 rpm, and its 293cc 72.0 x 72.0mm sin­gle makes 53.2 hp at the same revs. Be­cause two-strokes are sim­ple and light, these 228-pound­claimed off-road com­pe­ti­tion bikes will have ex­cel­lent per­for­mance.

What about smoke from lube oil? It is cut by up to 50 per­cent by load-ad­justed de­liv­ery from a pump de­liv­er­ing fuel-to-oil ra­tios from 80:1 to 110:1.

KTM knows that the per­for­mance of these bikes will ex­ert se­ri­ous pres­sure on heav­ier ex­ist­ing four-strokes. What mar­ket forces might that un­leash? It’s just a com­pe­ti­tion en­gine in the US for now, but a 53-hp, 230-pound street-le­gal dirt bike would be hard to re­sist.

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