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Posts Tagged ‘Automotive’

The Two-Cylinder Club

Today, I’d like to discuss a technology used in some vehicles called cylinder deactivation, or variable displacement. It’s a system used in some reciprocating engines that selectively deactivates some of the cylinders to improve efficiency and save fuel. This is generally accomplished by incorporating actuators that hold the intake and exhaust valves open so as to create an “air spring”, which has an equalizing effect on the overall combustion cycle of the engine. A relatively old technology, the closest predecessor to use a design like this was the hit and miss engine, which accomplished the task by holding the exhaust valve open. Several automotive makers have experimented with variable displacement models, having little success. Today, the concept is regaining strength with rising fuel prices and increasing environmental awareness. Some newer engines that were previously deemed “guzzlers” are now being redesigned with selective cylinder deactivation technology as a less costly alternative to a hybrid power-train. One example of this is the newer Chrysler Hemi engine, though I understand from talking to the owners of these that the Fuel Saver mode only kicks in above 65 MPH.

In yesterday’s post I talked about hybrid vehicles. It was my understanding that the engines in hybrid vehicles were directly coupled to the electric motor and deactivated cylinders to “shut the engine off” while still running on the electric motor. I was under the impression that the engine still rotated with the transmission without firing when not in use. I guess I can’t give the auto makers quite as much credit now, as I realize that the engines in hybrids behave much like they do in non-hybrid vehicles with automatic transmissions, stopping the engine completely and restarting with a smaller motor when needed.

I don’t really understand why all vehicles don’t have variable displacement technology. I mean heck, a car or truck only needs one cylinder to idle, not eight. The goal is to use the full potential of each cylinder. At idle or partial throttle there is a vacuum inside the cylinders; in other words, an engine only uses as much air as it needs to. This becomes a problem with larger, multiple cylinder engines because you get pumping loss, inefficiency resulting in low pressure at top-dead-center of the compression stroke. Of course, lower pressure equals lower efficiency. So if at idle we take away 7 of the 8 cylinders in operation, that one cylinder left over has to use much more of it’s maximum power output to maintain operation of the engine. I would gladly take away three of the six cylinders in my car if it meant better mileage, not like I need all six anyway. Diesels could accomplish this even easier, simply by shutting off the fuel to the cylinders that you want to cut, now we’re talking mileage.

I think that all automakers should start designing their vehicles with variable displacement technology. The changes are easy to implement, and the rewards would be great to the consumers. If they just invest a little more time in getting the system right, it could be huge for a form of propulsion that is on it’s way out anyway.

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Maybe I Was Wrong About Hybrids

I used to think that hybrid vehicles were a waste of time and energy. It made sense to me that a car that used an electric motor directly inline with the engine would never be efficient and would pretty much always be limited to the efficiency of the engine. If you’re new to the workings of gasoline hybrid technology, I’ll try to bring you up to speed. We’ve all heard of hybrid vehicles by now, they have a gasoline engine that is assisted by an electric motor and a bunch of batteries. There are two types of hybrids, series and parallel. Series hybrids have an electric motor mounted directly to (or basically on the same shaft as) the gas engine. The Toyata Prius and Honda Civic hybrids are the two best examples of these. Parallel hybrids have an electric generator mounted to the engine for the sole purpose of generating electricity for the electric motor that drives the wheels. Big diesel locomotives are a perfect example of this type of hybrid, which in my opinion, is the best type. For more information on hybrids, check out this Wikipedia page.

My big problem with hybrids was the fact that there have never been any commercially produced of the parallel type, they have all been series. This means that the engine still has to run at varying engine speeds depending on the speed of the vehicle, which is inefficient because gasoline engines are most efficient at a set speed and load. Upon researching the Prius, I came across the technical page explaining the Hybrid Synergy Drive. I now realize that vehicles that use this type of powertrain can function as both series and parallel hybrids. They accomplish this with two motor/generators that are connected to the engine and each other by a type of clutch and a continuously variable transmission (CVT). With this setup the engine can run independent of the wheels for the sole purpose of charging the battery, acting as a parallel hybrid. It can also function as a series hybrid by using one of the motors to assist the engine in acceleration and provide regenerative braking. The CVT in combination with the two motor/generators allows the engine to run mostly in it’s most efficient speed and load range. It seems to me, now, that the new technology added to these hybrids has utilized much more of the potential efficiency of gasoline engines, without the extra baggage of a full-out split parallel hybrid.

Despite this great new technology, I still don’t think that gasoline engines are the way to go. Diesel engines are far more thermodynamically efficient than their gasoline counterparts, we’ve known this for a long time. It’s all about the combustion process, compression ignition will always be more efficient because higher compression generally gives higher efficiency. A good running car with a four-cylinder diesel engine gets better mileage than one of the newer gas hybrids. I think we need a vehicle that features a small diesel engine coupled to one of the newer hybrid drive systems. That, in my opinion, would be the ideal hybrid.