New Engine Technologies Could Give New Life
Total Page：16
File Type：pdf, Size：1020Kb
New Engine Technologies Could Give New Life (and Larger Markets) to Ethanol Four-Cycle Internal Combustion Engines Robert Kozak [email protected] For Advanced Biofuels USA Automotive engines that run on gasoline and alcohol fuels are called 4-cycle spark ignition internal combustion engines. Internal The Challenge combustion (IC) simply means the fuel/air mixture is combusted (burned) inside the The use of E-85 (85% ethanol) fuel in “Flex- engine’s combustion chamber which is called a Fuel” vehicles has been limited by its 25 percent cylinder. A steam engine, by contrast, is an lower energy content than gasoline. When used external combustion engine. Water is heated to in engines designed for only gasoline, this produce steam in an external boiler and is then results in approximately 25 percent less mileage injected into the cylinder. for a fuel that costs as much as gasoline. So, unless E-85 costs 70% of the price of gasoline Four-cycle engines require the piston to go up (If gasoline is $2.65/gallon, E-85 would have to and down twice in the cylinder to produce one sell at $1.86/gallon.), there will continue to be power stroke. The four cycles (strokes) are: little demand for E-85. 1) intake-piston goes down, 2) compression-piston up, Engine-Design Solutions 3) power-piston down, and 4) exhaust-piston up. However, new engine designs that utilize the positive ability of ethanol to resist early ignition Four-cycle engines also use “poppet” valves at could regain much of this fuel economy loss. the top of the cylinder to regulate the flow of Possibly even more important, these engine intake and exhaust fuel/air mixtures. This type of designs produce increased low speed power engine is also called an “Otto Cycle” engine (torque) when using E-85. This torque increase named after Nicolaus August Otto its German could allow lower-cost E-85 engines to replace inventor. There are also two-cycle engines, that more expensive diesel engines in light-duty do not have such valves, but we’ll get into them trucks such as the Ford F-150. in a future article. The widespread use of these engines in both automobiles and light trucks could create a US market for cellulosic E-85 in excess of that mandated by the Renewable Fuel Standard (RFS). It is very important to realize that this market would be a sustainable, rather than an artificial market, since it would be based on vehicles having equal or superior performance characteristics to gasoline or diesel powered vehicles. Engine Design Basics Spark ignition means the fuel/air mixture is ignited by an electric charge. This requires a Because these new designs use some complex ignition system that can fire a sparkplug unfamiliar concepts, let’s start with the basics of at exactly the right time (a millisecond makes all automotive engine operation so we can the difference) no matter the varying operating understand, and appreciate, the advantages of conditions. these new power plants. This design also allows for lighter weight, less- nozzle at the local Exxon or Shell explodes expensive engine construction than required for spontaneously way before that type of diesel engines where the mixture is ignited by compression is reached. If you’d look at the compression. Compression ignition requires specification of the engine you’d most likely see much stronger and heavier cylinder wall a compression ratio in the range of 9-10. construction to withstand the higher compression. This means diesel engines are This spontaneous combustion caused by much heavier and much more expensive to extreme pressure is called by several different build. names: pre-ignition, dieseling (because it acts like a diesel), or knocking, because that’s what it Hence, if a lighter and less-expensive E-85 sounds like. What happens is that the fuel/air spark ignition engine could produce the same mixture ignites while the piston is still moving up low-end power with comparable fuel economy on the compression stroke. The resulting flame as a diesel, a car company would soon produce front pushes down against the onrushing piston it to improve their bottom-line. causing the piston to shudder and, in extreme cases, to break. Ethanol, Octane, and Engine Efficiency Automotive and fuel engineers have devised a Thermal Efficiency The absolute goal of all measurement for a fuel’s ability to resist pre- engine design is to extract the maximum amount ignition, it’s called the Octane Rating. Regular of energy contained in the fuel delivered to the gasoline has a rating of 87 while “high-test” has combustion chamber. This is called thermal a rating of 91-93. efficiency. However, this thermal efficiency goal is restricted by two realities: 1), the economics By now you’re asking, does any readily available and technology of engine construction, and 2) and relatively inexpensive fuel exist that has an the characteristics of the fuel being used. octane rating high enough to allow high compression ratios? Well yes. Alcohols do. E-85 The fact that automotive engines have to be has an effective octane rating of above 100. This both light weight and inexpensive to build is would allow compression ratios in the 13+ pretty straightforward, producing near perfect range. thermal efficiency in an engine that weighed two tons and cost $1 million dollars to build would be And you’re probably also asking, why not build worthless in the automotive industry. an engine that could benefit from this? Again, that would be relatively easy, if, and this is one The effects of fuel composition are probably not huge IF, you used E-85 100% of the time. If, as easy to understand. But let’s give it a try. however, you ever used straight gasoline or E- 10, the much lower octane would result in Thermal efficiency rises with the compression engine damage. ratio achieved in the compression stroke. Near ideal efficiency occurs when the ratio between So, we know automotive engineers can design the volume of the cylinder when the intake valve and build engines that maximize the closes at the bottom of the compression stroke performance of either gasoline or alcohols. We and the volume when the piston reaches the top also know current “Flex-Fuel” engines are is 17:1. This means the volume of the fuel and actually engines designed to maximize gasoline air is compressed 17 times. performance and not that of E-85. And that leaves us with the question of the decade: Octane Does your car or gasoline powered truck have a compression ratio this high? No. Why not? The gasoline that comes out of the Can automotive engineers design and build the engine is actually smaller than if Otto Cycle engines that maximize the performance of valve timing was used. In a hybrid application BOTH gasoline and alcohols? this loss of power is compensated for with an electric motor. Also, the Atkinson cycle is not The answer is yes ! really advantageous for alcohol fuels since very high compression ratios are not possible. New Multifuel Engine Designs Step Two: Gaining Back Power: The Miller From what you’ve read thus far, this “yes” Cycle answer would seem to involve a design that could vary the compression ratio based on the In the 1920s an American racing engine builder amount of ethanol in the fuel. In addition, the and designer Ralph Miller overcame the power design should also maximize the thermal deficiencies of the Atkinson cycle engine by efficiency possible from a compression ratio in adding a supercharger to the intake system. The the 12-13:1 range. Can these two objectives be supercharger increased the pressure of the met at the same time? air/fuel mixture coming into the cylinder during the shortened Atkinson compression stroke to Fortunately, by combining the current generation several times atmospheric pressure. This raised of computer engine controls involving multiple the amount of fuel and air available for sensors and actuators, advances in fuel injection combustion thereby increasing power and and turbocharging technology, and some thermal efficiency. previously overlooked historical engine designs, these goals can be simultaneously met. Since Miller was controlling all this mechanically (including the valve timing), maintaining peak Step One: Maximizing Thermal Efficiency, efficiency was virtually impossible. So, while The Atkinson Cycle Miller engines were very successful and won several Indy 500s, they were not suited to A late 19 th century English engine pioneer, production cars. James Atkinson, realized that to maximize thermal efficiency in an IC engine the power Step Three: Putting It All Together: The Eco- stroke had to have more time for combustion Boost Engine than for compression. His solution was a complex crankshaft system that produced a Starting in 2009, Ford Motor started putting shorter compression stroke and a longer power something called Eco-Boost engines in their stroke with the same length piston rod. Although cars. These small displacement mass-produced used in custom applications, this asymmetrical engines are generating over 100 horsepower 3 mechanical solution was never mass produced. (hp) per liter (61 inches ), a value reserved for expensive racing engines, with good fuel Now, however, both Toyota and Ford are economy. These figures show high thermal applying this Atkinson concept in their hybrid IC efficiency as well as a smooth, consistent burn gasoline engines. Instead of a complex that translate into increased torque (power). crankshaft though, through computer controls both manufacturers delay the closing of the intake “poppet” valves to create a shorter period of time for the compression stroke as compared to the power stroke. While this increases thermal efficiency through a longer period of combustion, it also decreases the available power since by decreasing the compression stroke, the size of At the Washington DC 2010 Auto Show, Growth Energy and Ricardo Engineering introduced a V- 6 engine of about 3.5L in size that could replace a 6.2L gas or diesel engine in a standard GMC Sierra 3500 pick-up truck.