2-Stroke Engine Warmup
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Home > Articles & Tips Index > Flying > 2-Stroke Engine Warmup

[Courtesy of Michael Kuper, December 1998 - originally posted on the IMAC mail list]

To Dan Ross and others that may have questioned my post regarding the warm-up and lubrication of two stroke engines, I offer this data.

I found research published through The Queens University of Belfast that examines heat transfer in the crankcase of two strokes. It details typical values obtained from a 65cc two stroke chainsaw type engine running at 4,000 rpm. This rpm was used for the example although peak power was actually recorded at 9,600 rpm. The values are obtained using the Annand model, there is continuing research as it is difficult to obtain empirical data due to the time lag of sensing devices.

Without all of the charts and graphs that accompanied the publication, it is hard to explain but I will attempt to simplify their findings. The engine tested was typical and had a primary (crankcase) compression ratio of 1.5:1. Ambient temperature was 68F, engine stabilized at operating temperature. Temperatures of crankcase gases are presented at two different points of the operating cycle.

Measurement 1 (M1) - height of crankcase compression (i.e. just prior to port opening), temperature of crankcase gases is 248F.

Measurement 2 (M2) - peak of intake vacuum (crankcase pressure at .6 atm), temperature of crankcase gases 140F.

So it goes like this, the air enters the carb at 68F, the temperature drops in the intake and is introduced to the crankcase. Convection heat transfer from internal components and residual heat from the prior stroke raise the temp to 140F (M2). On the downstroke of the piston, compression raises the temp further to 248F (M1). As the port opens and pressure falls, the temp falls and another cycle begins. Although much of the temperature rise is due to compression, heat from internal components is also an important aspect. Further studies by QUB determined that crankshaft size and crankwheel mass affect the outcome of the heat transfer. Obviously radiant heat and convection heat transfer are the reason for this.

Another point to consider, not all of the fuel charge introduced into the crankcase enters the cylinder with each stroke. Only a portion reaches the cylinder with each cycle. Picture three people standing in a door way. The first in line is at the transfer port, the last in line is at the carb or inlet duct. As the first person enters the door, another takes his place at the back of the line. The one in the middle goes through this temperature change a couple of times before entering the cylinder.

Back to engine warm-up. Because heat transfer from the internal parts is an important component of this heat cycle, you should allow your engine a warm-up period. When the rods and crankshaft are cold (near ambient temperature) as the intake charge is introduced and heated, some of the atomized fuel droplets "cling" to these colder parts. The atomized liquid fuel in effect washes these parts with a gas/oil mix. Oil diluted in gasoline is not an ideal lubricant. Once warm, when the gases follow the temperature cycle outlined above, the atomized fuel begins to vaporize as the charge is heated. The oil precipitates out of the solution and lubricates properly. 248F is not the absolute boiling point for gasoline, so not all of the fuel charge is completely vaporized until it reaches the cylinder. But remember that the charge goes through this temperature cycle a couple of times and much of the oil is deposited before it enters the cylinder. This thermodynamic stuff is hard to imagine, but the data is real (no Dan, I do not make this stuff up).

Bottom line, if you care about your engine, give it 30 seconds or so at ~2,500 rpm before you run it up. You'll enjoy your engine a lot longer.

Another note - I still can't find any data that supports an advantage to running high oil/gas ratios. In fact the opposite is true. Even Amsoil states on their web site that they recommend 50:1 "for high performance applications".

Mike Kuper

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