Propwash October 2019 web.pub

This is the geometric compression ratio, but the effective compression ratio is reduced by the time the exhaust port is open. Because of the effect of this exhaust timing, I don’t know of any standard recommendation for compression ratios. Small, racing motorcycle two strokes can run geometric compression ratios of 19 to 1 on straight methanol. Racers, stuck with an engine and head combination, can use head shims to vary the compression to change the ignition timing over a narrow range. Most engine builders use the cut and try method of varying the volume of the head button to find the best result. Liability and Accident Coverage (Continued from page 3) As an example, if you are injured at a NAMBA event, such as cutting your hand on a spinning propeller, go get medical treatment. Whatever amount is not covered by your personal health policy will be covered by the NAMBA policy. But, your personal policy will be expected to pay first, then what is not covered will be sent to our insurance company. Typically, personal coverage will pay almost everything, but in those cases where a member has no health insurance or limited coverage, NAMBA insurance is there. The reason behind providing accident insurance was that some members without insurance felt they had to quit NAMBA as they could not afford an accident. Over the years, NAMBA has relied on these insurance policies to protect our members from lawsuits and expensive medical bills. Some insurance claims have been small and some quite large. We need each member to think safety first. Just because we have the coverage, does not mean we should take unnecessary chances. I have personally seen two serious injuries sustained by NAMBA members over the years. Both were extremely painful and while the boaters recovered fully, those experiences make us realize how dangerous it can be if we ignore safety concerns. The Travellers Liability coverage and the AIG Accident policy are in place for all current NAMBA members, including Single Event Members. The coverage does not apply for an accident you have running a model boat in your backyard swimming pool. But, when you are at a NAMBA event, you have the best insurance coverage provided by any model boating organization. Your membership card is proof that you are a NAMBA member and that the insurance coverage is there to protect you. Make sure to have your membership card with you at all NAMBA events, as the CD may require you to produce that card to prove you are a member. I hope you never have a serious accident. Safe NAMBA boating. The Nitro Engine: Part 3 By Lohring Miller NAMBA Safety Chairman Again, according to a conversation I had long ago, the second most important part of the nitro engine after piston and sleeve fit is the cylinder head. This is largely due to the nature of glow ignition. A glow plug ignites the fuel and air mixture when it is compressed enough that the hot platinum element causes ignition. A lot of factors determine when this happens. The process is actually the same as preignition (NOT detonation) in a spark ignition engine. Diesel engines also use a heated plug to help ignite the fuel when cold. How rich the mixture is as well as the methanol to nitro ratio effects ignition timing. Other fuel elements like propylene oxide also effect ignition. However, with a consistent fuel mix, the ideal mixture can be adjusted easily for best power. That leaves the head’s compression ratio as what sets the ignition timing. The factor that determines this is the head volume. The ratio of the head volume to the total cylinder volume (head volume plus cylinder swept volume) is the compression ratio. As an example in a 26 cc Zenoah engine (actually 25.4 cc swept volume), the head volume is around 2.4 cc. That makes the compression ratio 25.4+2.4 divided by 2.4 or 11.6 to 1. PROPWASH 4 Head Buttons October 2019 A point of flame starts to burn at 10 degrees before TDC. There is another factor that determines the character of combustion in the cylinder. The head consists of two areas, the squish area and the central combustion area with most of the volume. The squish area is very close to the piston at top dead center. This means most of the mixture is in the central area, close to the plug, and can be ignited quickly. The squish also forces the mixture into the central area as the piston rises, causing turbulence that enhances combustion. This close fit also cools the trapped mixture outside the central area, discouraging detonation. This is how the squish works: Mixture flow as the piston approaches TDC. The sharp edge between the squish area and the central volume encourages the flow across the top of the piston.