Where Do These Emissions Come From?

Transcription

1 Outboard Emissions 2008 marks the tenth anniversary of the beginning of exhaust emission regulations for outboard engines. Recreational marine engines came to the emission regulations party quite late, when you consider that car regulations started in the 1960 s. It s not that the industry wasn t concerned about emissions, because studies into outboard engine pollution of the marine environment actually began way back in the 1950 s. They mostly concluded the effect on marine life was negligible. Some studies even indicated an improvement in marine plant growth in areas of high boating populations, probably due to increased oxygenation of the water through boating activity. However the real need for pollution controls was not in the marine environment, where there are many natural organisms that help clean house, but in the air borne environment which has to rely only on sunlight, wind and rain. By the 1960 s many big cities in the US and Europe could not achieve their clean air targets regularly, so the emissions regulations started, firstly with road transport and later with industry. The smaller polluters like marine engines and off-road vehicles had to wait until the larger polluters (judged by the percentage of their contribution to the overall effect) had been effectively regulated. By the late 1980 s most road transport was onto its second or third generation of emission controls and big gains had been made in air cleanliness, so the time had arrived for the smaller engines to come under the emissions spot light. Luckily the marine industry could learn a lot from the automotive industry, and it was anxious to avoid the lengthy legal battles and awful products that resulted from some of the early automotive emissions rules. Industry bodies like ICOMIA (International Council of Marine Industry Associations) and the NMMA (National Marine Manufacturers Association, USA) took a leading role in developing standards for pollution measurement, using the resources of their marine industry members. This was one of the very rare occasions when engineers from rival manufacturers got together to develop some real-world standards that would accurately represent exactly how recreational boaters used their craft and how best to measure emissions from engines. The US government also wanted to avoid the conflicts of the past and became actively involved with the industry in developing the legislation that would result in big reductions of pollutants, while not decimating the industry. Two key standards were developed that are still in use today around the world. How to measure marine engine exhaust emissions (ICOMIA 34-88), and the Duty Cycle (ICOMIA 36-88) that specifies which speeds and loads are to be used. Emissions measurement requires a state of the art, computer controlled dynomometer test cell.

2 These cost in excess of US$1,000,000 each to build and more than US$30,000 each per year to maintain. They need to have equipment that can measure accurately down to 0.1 parts per million for HC and NOx, and to 0.01% for CO, CO2 and O2. Each full engine test generates more than 130,000 pieces of data that must be analysed, hence the need for some very powerful (and expensive) computer equipment. Manufacturers like BRP (makers of the Evinrude brand of outboards) will require 3 or 4 such cells in order to test sufficient engines, and we are unlikely to see one outside of the engine manufacturers factories. The Duty Cycle used, or how long we spend at each speed and load point, was developed by analysing more than 20 years of data gathered by the engine manufacturers and universities studying recreational boating. It is based on 5 mode points from idle to wide open throttle (WOT). When most people first see this 5-mode duty cycle the normal reaction is there s no way I spend 40% of my time at idle, however now that most modern engines have a management system that records usage profiles, it has proved remarkably accurate. Not everyone fits the duty cycle, but it s a good fit for average recreational boating, where most of us go. The first country to adopt outboard emission laws was the USA. The US EPA regulations began on 1998 model outboard engines, using a sliding scale that aimed to reduce emissions 75% by Corporate averaging is allowed so that building low emission models gets the manufacturer credits to allow some high emission models. In the beginning building one low emission model allowed enough credits for about 20 high emission models, but by progressively tightening the regulations each year, by 2006 that had been reversed. California s Air Resources Board (CARB) decided, due to their more pressing air quality concerns, to accelerate the process and brought in their own regulations. These required 2006 EPA levels to be achieved by 2001, for all new engines sold in California, plus a further reduction for 2004 models, and then another for Models sold in California would then be more than 90% lower in exhaust emissions than 1998 models. To make these California models easily recognizable to consumers the CARB regulations required a Star label to be attached to each new engine, one star for engines meeting the 2001 rules, two stars for 2004 and three stars for This very simple and easily recognised system of star ratings has now been adopted by other countries around the world.

3 In 2005 the European Union began their outboard engines exhaust emissions regulations. By 2007 some 75% of the world outboard market (North America and the EU) had emission regulations in place for all new engines, so building old (high emission) carburetor models is rapidly becoming no longer economic and they are starting to be dropped from manufacturer s line ups. What Emissions? The major problem is air pollution, and the major culprits are Hydrocarbons, usually abbreviated as HC, (sometimes also called Volatile Organic Compounds or VOC); Oxides of Nitrogen, usually abbreviated as NOx; and Carbon Monoxide (CO). HC+NOx and the right amount of Sunlight cause the formation of ground level Ozone (O 3 ). High level Ozone (in the Stratosphere) protects us from the sun s more harmful radiation but low level ozone is an irritant to our eyes and respiratory system, plus it s a highly reactive chemical that can affect surfaces, crops and vegetation. CO is not so much an urban pollution problem, but a more of a personal one. CO is highly poisonous to humans and is invisible and odourless, so it s very hard to know when it s about to be a problem. CO can most easily affect us when we operate internal combustion engines inside confined spaces, or on vehicles used at low speeds when we can t rely on vehicle movement to keep the exhaust away from us. Boating emissions rules are now starting to include CO. The chart below shows how the rules have affected HC + NOx from outboard engines over the last 10 years, using a 90HP outboard engine as a typical sample. The introduction of oil injection in the 1980 s provided a small improvement, but once the US EPA rules came in 1998, the big reductions started. By the late 1990 s when 4-stroke and direct injection 2-strokes started to get popular, emissions had dropped by 75%. By the time 2009 models arrive it will be over 90%.

4 While EFI 4-strokes and DI (direct injection) 2-strokes can get the HC and NOx down, CO is a little harder to deal with. The chart below shows relative CO emissions, again using a 90 HP engine as a sample. The best outboard engines for low CO emissions are the Evinrude E-TEC direct injection and some of the latest 4-strokes with variable valve timing and closed loop control. The chart above covers emissions across the Duty Cycle range, but CO emissions at idle are the biggest risk factor because most 4-stroke engines have very high concentrations at idle when the boat speed is low or zero, so there is no natural movement to help with ventilation. The National Institute of Occupational Safety and Health (NIOSH, USA) lists the lethal limit for CO as 1200 parts per million. As this NIOSH chart shows the idle exhaust concentration of most common engines easily exceeds that value but a large margin. This is why CO regulations are starting to appear and why special exhaust plumbing on slow boats (like house boats with generators) is getting a lot of attention. Direct injection 2- stroke engines like the Evinrude E-TEC have very low CO emissions primarily because they have an excess of air available while scavenging the cylinder, plus they use stratified combustion at low speeds where only a small portion of the combustion chamber is used. The remainder of the chamber contains only air or exhaust gas from previous cycles.

5 Where Do These Emissions Come From? Firstly, if the process of scavenging the exhaust from the previous combustion cycle is not perfect, then some of the new charge gets lost out the exhaust. Conventional crankcase scavenged 2-stroke engines are the worst here. To get everything done in just two piston strokes means the inlet and exhaust ports are both fully open together and it s possible for up to 40% of the fresh charge to be lost out the exhaust. However when the fresh charge is only air, as it is in direct injection engines, then there s no problem, and in fact it becomes a benefit as the free flow of air ensures good cylinder scavenging and the additional oxygen promotes combustion of any remaining fuel in the exhaust system. The graphic at right shows a 2-stroke engine when the piston is rising and about to close the exhaust port. In the carburetor engine on the left, some of the fresh mixture has escaped into the exhaust port, leading to high exhaust emissions. In the DI engine on the right fuel injection has started but will not reach the exhaust port before it closes. Only fresh air is lost out the exhaust port. Next, there s the process of combustion. Most of us can remember in chemistry class that if we mix air and a HC fuel in a process called combustion, we should get out just heat, water and CO2 (Carbon Dioxide) all relatively harmless, so where do the pollutants come from? If we could freeze the combustion process, like in the graphic of a combustion chamber below, we d be able top see why. We break down the liquid fuel into tiny droplets which quickly vapourise into a highly combustible gas, mix it with just the right amount of air so we ll have the right number of oxygen molecules to match the Hydrogen and Carbon, then we start the process with ignition by electric spark. This creates a flame front that rapidly travels across the chamber as it consumes the fuel and this is where the problems come. As the flame progresses the gasses behind it are very hot and expanding, while the gasses in front are relatively cool and now being compressed. That changes their composition and willingness to mix with oxygen. Plus there are some pockets of exhaust remaining from a previous cycle, and some liquid fuel that did not vaporize, or has condensed on the walls. Cylinder head The end result is some small pockets that do not Piston combust properly, or only partially, leaving some unburnt fuel (HC), some partially burnt (CO) and some unintended reactions from the very high temperatures (NOx) and inability of all the oxygen and HC molecules to find each other. What is really needed is a way to get all the combustion to happen simultaneously (not via a flame front). One process that promises to do this is called Homogenous Charge Compression Ignition (HCCI) and is currently being studied by most engine makers. If you re thinking that sounds a bit like continuous detonation, you re on the right track, although the engine is naturally designed to handle it.

6 GM have already demonstrated vehicles that can maintain HCCI at steady highway speeds, but it s still a long way off from everyday production. How Much? If you could capture the emissions from typical engines over an hour s running, what would they look like? Again if we use 90 HP (67kW) engines as an example, and take the major component of emissions, the HC+NOx component, we can illustrate by calculating the emissions in litres. Each engine is tested to the ICOMIA duty cycle for an emissions result in Grams per Kilowatt, per hour. Using the same duty cycle we can calculate the average power output (23HP or 17kW) over the same duty cycle, then the emissions per hour are Engine Type HC+NOx Emissions Carburetor, pre-mix 2-stroke 5.15 L Carburetor, oil injection 2-stroke 4.13 L Direct Injection 2-stroke 0.92 L Evinrude E-TEC Direct Injection 0.29 L Typical HC+NOx emissions over 1 hour at ICOMIA duty cycle, 90 HP engines It s a scary thought that your typical old-fashioned 10 year old carburetor 90 HP Outboard might actually be emitting 5 litres of pollutant for every hour of operation, but that s the reality. Or if you like, about $7 of your money is wasted every hour. Emission Strategies When the current emissions regulations were being formulated the rule makers did not want to restrict design to any particular technology, so the rules do not specify any type of engine, just the maximum amount of exhaust pollutants allowed, and the test methods. Manufacturers are free to explore any technology they feel will do the job. The easy way to go was to adopt 4-stroke engines instead of the carburetor 2-strokes that had been common in outboards for 80+ years. A 4-stroke engine only has the intake and exhaust valves open together for a very small percentage of the cycle time, and even then the valves are barely open, so there s very little opportunity for the fresh charge to be lost out the exhaust. Plus 4-strokes have been popular in cars and trucks for 100 years so there s lots of knowledge available on how to build them. Most outboard manufacturers went with 4-strokes, at least in the beginning, but they come with penalties. Having a power stroke only every 4 strokes makes it harder to get the required power level for a given size engine. The extra complexity of camshafts, valves, springs, much larger cylinder heads and wet sump lubrication system adds a lot of weight, and boat performance is very sensitive to power to weight ratios. There were very good reasons why outboard motors had stayed with 2-stroke induction for those 80+ years. What was needed was a way to make the 2-stroke engine a low emission one and retain their power and light weight - enter direct injection. Not a new idea on piston engines, but new to mass production and not very practical without the electronics revolution of the late 20 th century. By using the same sort of electronics that makes the home computer and the mobile cell phone possible, engine control has advanced in leaps and bounds, making direct injection engines mass producible. The first direct injection production outboard motors appeared in the late 1990 s. As the charts above show they had much better emissions than any carburetor or EFI 2-stroke, but not quite as good as an EFI 4-stroke. The injectors used electro-magnetism or compressed air as the force to push fuel through a very small opening nozzle breaking it into many tiny droplets, in the combustion chamber. These early DI systems were single action, or only driven in one direction. A spring returned the injector

7 plunger or armature to get ready for the next injection. At high engine speeds (about rpm) it required around 180 degrees of crank rotation to get enough fuel into the cylinder, so there was still some fuel entering the cylinder while the ports were open and hence some could be lost out the exhaust. The Evinrude E-TEC injection system is a double acting type where the injector is driven in both directions. This generates higher pressures in a shorter time, reducing exhaust emissions to equal or below 4-stroke levels while retaining 2-stroke performance. See attached illustration for the exact sequence of events during one E-TEC injection cycle. Clean Water Test While nearly all the current legislation is aimed at air pollution, studies also continue into the effects of the latest engines on the marine environment. In a recent test conducted by a US water supply authority, two 90 HP modern low emission engines were each operated in a 12,000 litre water tank, at the ICOMIA duty cycle for 30 minutes. The tank was drained and cleaned between each engine test. The US EPA laboratory standard tests for water cleanliness were then used to analyse the water sample from each test. The results indicate today s low emission engines are very good for the marine environment, as there was zero detectable oil, grease or unburnt fuel in the test samples. The future is looking good for the modern, low emission outboard motor. Technology like the Evinrude E-TEC is retaining traditional boat performance, with very low noise levels, turn-key starting in any weather, plus the desirable benefit of better fuel economy when we do not waste fuel creating emissions.

8 The sequence of events in an E-TEC injector. Only the lighter components (electrical coil and plunger) move. The heavy components like the magnets are stationary. The electrical pulses are represented by the oscilloscope like trace at the upper right, with an arrow at the current location. A positive electrical pulse has entered the coil, which quickly generates a magnetic field opposing the stationary magnets. This forces the coil and plunger to move very rapidly to the right, compressing the fuel behind the nozzle until it opens, spraying tiny droplets into the combustion chamber. A fraction of a second later the positive pulse is turned off, but inertia in the moving components continues injection for a moment longer. A reverse polarity pulse is sent to the coil. This causes an opposite polarity magnetic field in the coil and an immediate change of direction. The coil and plunger stop and then start the return journey. Injection stops as the pressure behind the nozzle drops off. A second reverse polarity pulse is sent when the coil and plunger are about to come to a stop at the end of the housing. This is a cushioning pulse that prevents the moving parts from bouncing, so they are quickly ready for the next injection cycle. The entire process takes only seconds, or about 90 degrees of crank rotation at 6000 rpm.