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Dr. Horizon gitano CBT
The secret that everyone knows is that two-stroke engines are dirty. They are gas hogs, noisy and smoke more than a burning tire. Because of increasing concern over fuel consumption and emissions, the sales of new two-stroke powered equipment is being banned in country after country for all except the smallest applications. Even four-stroke powered grass trimmers are starting to show up. So what’s so secret?

 

Dr. Horizon gitano CBT

Diesel engines are often referred to as compression ignition engines because compression of the air in the combustion chamber raises temperature above the ignition point of the fuel. In gasoline engines the spark initiates combustion of a more or less homogeneous air fuel mixture at the spark gap. In diesel engines, however, the fuel isn’t even introduced into the combustion chamber until shortly before top dead center, well after the air is hot enough to burn the fuel. And that’s how the short version of diesel combustion goes: the fuel is sprayed into the combustion chamber where it spontaneously burns in the hot, compressed air. As with most things in life, the simple version lulls us into a false sense of understanding.

 

Dr. Horizon gitano CBT

Laws exist for a reason, or so we are told, and some of them actually make sense.  The real problem with laws is they don’t actually work. A speed limit, for example, doesn’t actually keep you from speeding. The basis of many laws is the so-called “golden rule”: do unto others as you would have them do unto you. I prefer “law of reciprocity” which can be stated as “you aren’t allowed to do anything to the next fellow that he isn’t allowed to do to you”, and of course visa-versa. This is important to keep in mind when making other, more specific laws. Smoking is one of those contentious issues which is fairly easily resolved via the law of reciprocity. If we all have the right to fresh air, and we also have the right to smoke, then am I allowed to smoke in a bus full of non-smokers? If exercising my right infringes on the rights of others, then that is where my right ceases to be a right. No, I don’t have the right to smoke on the bus; if I want to smoke, I’ll have to do so somewhere where it does not infringe on the right of others to breathe fresh air.

I was recently involved in the drafting of standards for electric motorcycles, along with a dozen or so representatives from government, industry and environmental organizations. One of the important concepts I tried to apply to the process of generating a standard was “Only do what is necessary, but do everything that is necessary.” This, I believe, helped keep us focused on the most important issues

 

Dr. Horizon gitano CBT

We all know that our car’s fuel consumption is related to the position of the throttle: more throttle, more fuel consumption, right? Well, actually it’s a little more complicated than that, and rather counter intuitive. In fact in some cases more throttle will actually give you less fuel consumption.

So an engine running at 3000 rpm and 100Nm of torque is producing the same amount of power as when it is running at 6000rpm and 50Nm of torque. These two situations, however, will have very different fuel consumptions. For a small engine 100Nm of torque may coincide with full throttle, so running at 3000 rpm the throttle has to be fully open. In this case air can pass into the engine with very little resistance from the throttle. At 6000rpm and 50Nm or torque the throttle will be partially closed. The engine has to work to suck air past the partially closed throttle. This results in increased fuel consumption as some of the engine’s power is devoted to pumping air past the throttle. Additionally at high speeds more power is lost to friction. These two effects, higher pumping losses at “part throttle” operation (called throttle losses), and higher friction result in a higher fuel consumption at the higher speed, even though the power output is the same as the lower speed.

 

Dr. Horizon gitano CBT

The fuel injector is basically an electronic solenoid valve. You have a coil of wire (ie. the solenoid), and a “pintle” or rod made of steel. The pintle partially penetrates into the solenoid, and a spring pushes the pintle out of the coil. When current is passed through the coil it creates a magnetic field attracting the pintle, pulling it further into the solenoid as it compresses the spring. One end of the pintle covers a fuel flow orifice, opening when current is applied, allowing fuel to flow out. Inwardly opening pintle valves are pulled in towards the body of the injector opening the fuel flow orifice from the “inside”. There are also outwardly opening fuel injectors where the pintle valve is “pushed” away from the body of the injector to open. In this case the pintle end may look like a small poppet valve or golf tee resting in the fuel flow orifice. Pushing it out allows fuel to flow around the head of the valve.

 

Dr. Horizon gitano CBT

When we burn fuel in internal combustion engines the oxygen in the air is reacting with the fuel to create (mostly) carbon dioxide (CO2), water and heat. Nitrogen in the air goes through the engine mostly unchanged, with a little being oxidized to NO or NO2 (collectively called NOx). Also because there is no way to completely mix the air and fuel homogeneously we will always generate some partially burned fuel, resulting in hydrocarbons and carbon monoxide (CO). If we run the engine lean, with less fuel than the amount of air available can combust, we will have excess oxygen, and much less CO as there is spare oxygen available to completely burn the CO to CO2. If the engine is operated rich, with more fuel than the available air can burn both CO and hydrocarbon emissions increase quickly, and O2 is reduced, but does not go to zero. Although there is more than enough fuel to completely consume the oxygen, it is impossible to get the air and fuel to mix completely, so there will always be some small pockets of O2 which are not burned. Near the chemically correct air-fuel ratio (14.7 to 1 by mass) O2 exhaust gas concentration increases rapidly to the lean side and CO decreases, while to the rich side CO increases rapidly and O2 decreases. Therefore if we can measure the amount of O2 in the exhaust gasses, we can infer the air-fuel ratio, and determine if we are operating lean or rich.

 

Dr. Horizon gitano CBT

One of the questions I get asked most often is which is the best motor oil to use. When you go to a shop to buy oil for your vehicle, you are confronted with a wide variety of oils to choose from, ranging from expensive “synthetics” to cheap “brand X” oils. Which oil is best for you depends a lot on the vehicle and driving pattern and how regularly the oil is changed. Motor oils fulfill a number of roles in an internal combustion, some of which are engine type specific. This is why there are so many different kinds of oils available.

 

Dr. Horizon gitano CBT

As motor heads we all know the basics: the piston compresses the air fuel mixture, shortly before TDC the spark ignites the mixture and it burns pushing the piston down for the power stroke. The details of what happens during this combustion phase are pretty fascinating.

First off the air and fuel is almost never completely homogenous: there are pockets of exhaust gas, pockets of air, and even droplets of liquid fuel when burning gasoline. Second, this spotty air/fuel mixture is in motion swirling around the combustion chamber in an irregular pattern with lots of turbulence to confuse things.  Each combustion cycle has slightly different air flow, and although the bulk pattern may be the same, the fine details of exactly how each parcel of air is moving is completely different. When the spark fires in a pocket of recirculated exhaust gas, it has a hard time igniting. If a fuel rich pocket of air happens to pass the plug just as the spark fires, it has a much easier time igniting.

 

Dr. Horizon gitano CBT

Back in the day when cars had carburetors, we expected to see a bunch of complex mechanical parts around the throttle. There were venturies and fuel jets spraying fuel into the air flow, choke valves, vacuum pressure lines, fuel lines, electrical fuel cutoff valves, vacuum controlled throttle props, and etc. But once we switched over to electronic fuel injection (EFI) the carburetor was replaced by a relatively simple throttle body (and of course the fuel injection system down stream of the throttle body). Basically this is a very basic device; it is just a “butterfly valve” that lets you control the amount of air entering the engine, and in some simple EFI systems that’s it. Well, almost.

 

Dr. Horizon gitano CBT

Growing up in a rural area of Colorado where the cars are cheap and the roads unpaved we were accustomed to “play” with our cars. The roads were wide and pretty much empty, so we had a lot of fun skidding, doing doughnuts (nobody called it drifting back then) and generally abusing our tires and cars. Most of the cars we had were front engine, rear wheel drive which is the worst configuration for straight ahead traction, but that just made it all the more fun. You never simply went around a corner; the goal was to leave the biggest skid mark and through up as much gravel as possible. Ideally you’d keep the car in a skid (yes, this means drifting) up to the next corner, perhaps a half a kilometer ahead. We spend hours “honing our skills” in empty parking lots, dirt roads and in the winter on snow and ice. Needless to say we spent more on tires than on the cars them selves, except when we’d crash. For the most part a crash simply meant that you trashed your car, and either had to fix it, or spend another 500$ for a “new” one (we were in high school remember, so our cars were the dented, rusting miss-matched body paneled wonders you’d expect to find at a demolition derby). Of course this was back in the day before Anti Lock Breaks (ABS) and traction control systems, so we had “total control” over our cars, and we liked knowing just where the back end would break away in a turn.

 

Dr. Horizon gitano CBT

SOHC, DOHC, 2 –vale, 3-valve 4-valve and VVT? What is up with the valves in our engines? All of these terms are referring to the intake and exhaust valve systems which allow our engines to breathe in air and fuel, and exhale combustion products. Although there have been many advances in valve technology over the years, the valves in our engines are still very similar to those of 100 years ago. Yes, although 4-valves per cylinder might sound high-tech, there have been SOHC 4-valve per cylinder engines for a century now.

So what is all the hype about?

 


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Focus Applied Technologies specializes in engine and vehicle technologies. We develop and use a wide range of tools in our ongoing alternative fuels R&D work. Selected products are offered for commercial sales.

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Our main product lines include engine and chassis dynamometers, engine test systems, engine and vehicle testing services, dynamometer controllers, electronic fuel injection systems and other control systems. In view of our unique expertise, we do often requested to deliver Technical Training workshops as well the wide range of engineering consulting services for various industries, academia and government institutions.

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