Two-stroke emissions

Economical E-TEC

The plastic bottles (pictured) represent the amount of pollution discharged from two 115hp two-stroke motors in one standard test hour. The single bottle is E-TEC. They are part of a dramatic demonstration by BRP/Evinrude intended to show the difference between their older carburetted technology compared with current direct injected technology on two-stroke engines.

 Although the company no longer produces carburetted two-stroke engines, there are still plenty of them on the water, and new units are still available.
 
The demonstrations (in Melbourne and Sydney) were presented by Gary Fooks an academic from Queensland University who has been gathering and analysing EPA audited data on all outboard engines available in Australia. In addition to his professional qualifications, Gary is a life-long boating enthusiast, too.

As concerns over the environment grow, emissions regulations are only going to become tighter. Smart companies are doing all that they can now to prepare for intensified scrutiny and the stronger regulations that will result.

Additionally, consumers concerned about the environment are demanding greener products and companies are taking slightly different approaches to meet demands. Last issue we looked at air assisted direct injection, this time we take a look at the BRP/Evinrude approach to squirting fuel directly into cylinders.

Just for the moment, we'll leave aside the technical details of the Evinrude direct injection system and summarise the data presented. The demonstrations weren't about criticising opposition products, so the company used two of its own engines (below and right). Both were rated at 115hp, but the Johnson was carburetted while the E-TEC had direct injection and advanced engine management.

I WHAT?
Fuel consumption and emissions testing of the engines were conducted according to the 5-mode ICOMIA duty cycle (the documentation for this and all other ICOMIA standards can be downloaded here).

In summary, though, the test period is divided up as follows: six per cent of the total test time is spent at maximum rated revs (Wide Open Throttle), 14 per cent at 80 per cent revs, 15 per cent at 60 per cent revs, 25 per cent at 40 per cent revs and 40 per cent at idle.

In a recent article, BRP national training manager, Paul Dawson, suggested that people are generally surprised at the length of time specified for idle. He says, however, that ICOMIA specifications really are representative of the actual boating habits of the broad range of operators.

Start up at the beginning of the demonstration was a lesson in itself. The smoke pouring out of the carburetted engine was everything we remembered the two-strokes of yesteryear to be, and the noise familiar to anyone who's ever owned a previous generation two-stroke.

The problems of carburetted two-strokes are many and excessive fuel consumption is one of them. During testing, the 115hp E-TEC consumed 8.67lt/h while the Johnson used 13.06lt/h. Over a three-year operating period (300 hours) with a nominated fuel price of $1.70 per litre, fuel for the carburetted engine ends up costing $2239 more than the E-TEC.

The initial purchase price for the E-TEC is $15,569 compared with $12,027 for the Johnson and cheaper prices like this have been luring buyers into carburetted models. However, the $3542 price difference changes when the cost of fuel is considered.

The company demonstrated that depreciation changes things even more. According to data from Glass's Guide, the E-TEC depreciates at 19.8 per cent per annum, while the Johnson does so at 25 per cent. Applying these figures to the purchase prices and then adding the fuel savings puts the E-TEC in front by $1077 as far as value for dollar is concerned.

The E-TEC is marketed strongly as not needing dealer servicing for the first three years of operation. This further reduces the operating expense of the new technology.

Gary added: "We haven't calculated the interest expense if you took out a loan, but you get the general idea. Ever since petrol went over $1 per litre, a carburetted two-stroke hasn't been the bargain it appears."

So, overall, buying the newer technology actually works out to be less expensive. Then there's the environmental cost.

HOW WOULD YOU FEEL?
US EPA emissions testing records hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). Here's where the plastic bottles in the photographs (main previous page and opposite top right) come in.

The larger group represents the 13.33kg of HC and NOx emissions created by the carburetted Johnson in one hour. The lone bottle represents the 1.07kg of HC and NOx emissions from the E-TEC for the same period. As Gary asks in relation to the Johnson emissions: "How would you feel about pouring all that out on the water behind you as you spend an hour motoring out to your destination?"

Outboard exhaust gasses pass through the water before dissipating into the atmosphere, and so creates two types of pollution. HC emissions consist of unburned hydrocarbons from fuel and oil. The molecular structure of these varies and they can form a number of compounds that damage the marine environment. NOx increases the acidity of water. Thankfully, CO emissions aren't water soluble, so they don't create water pollution, neither does CO2, but they still affect air quality and global warming.

We asked Gary which of these pollutants is having the worst effect. "It's a bit like asking which is worse for you, too much sugar or too much salt", he said. His point, of course, is that all emissions are bad and have to be cut back as much as possible.

Gary further pointed out that if we swapped all carburetted two-stroke engines (including lawn mowers and the like) for direct injection technology tomorrow, we'd reduce toxic emissions by 1.5 million tons annually and greenhouse gasses by one million tons. Although that's only about 0.2 per cent of all greenhouse gasses produced in Australia annually, it's still significant because the 550 million tons representing the total amount comes from all activities including coal-fired power generation, motor vehicles, livestock, all forms of industry, etc. Considering the wide variety of greenhouse gas emitting activities undertaken in the country each year, and the bulk nature of much of the CO2 created, any single activity that even registers is significant.

LOOK IT UP
Although we covered some aspects of direct injection previously (more here), there's still a great deal to say.

The Evinrude direct injection system on the E-TEC engine range isn't air assisted, so it achieves ideal droplet size by means of increased fuel pressure, which is the conventional approach. However, the injector is anything but conventional.

Because the main body of an injector in a direct injection engine is so close to the combustion chamber, fuel within it would become very hot if it wasn't circulated. In E-TEC injectors, a generous amount of fuel is circulated through the injector to prevent this. In fact, it's one of the main differences that distinguish an E-TEC injector from a standard type used in a port injection system. Although fuel in a port injection system is circulated through the fuel rail, it doesn't pass through the injectors except when it's actually being injected.

The high-strength magnets used in E-TEC injectors are quite heavy so the designers created a system in which the lighter coil moves instead of a magnetically permeable core as in a stationary coil. The E-TEC coil is attached to a plunger that increases fuel pressure behind a pintle/needle in the nozzle causing it to open and admit fuel to the combustion chamber.

STRONG CURRENTS
Conventional injectors are closed with a spring, but E-TEC types apply reversed current to the coil to stop the components and get them moving in the opposite direction to close the injector. There is still a spring in the E-TEC injector, and it does assist the reverse current in closing the injector, but it's too slow to do the job alone at higher revs. It's the reverse current that makes the process so fast. Just as the coil is coming back to the at-rest position, another brief burst of current is applied to the coil to cushion its return. Having engine management and electrical systems capable of doing things this way is what's brought direct injection systems to their full potential.

Controlling the internal components with current rather than a spring means the injector can complete its opening and closing cycle in about 90° of crankshaft rotation, even at engine speeds as high as 6000rpm. In turn, this means the fuel injection event can be timed to occur when the piston has fully covered the exhaust port. Obviously, this leads to a dramatic reduction of HC emissions, but that's far from the only advantage of direct injection.

In a conventional engine the fuel/air charge is homogeneous, or fully mixed, within the combustion chamber. This is good at WOT when the maximum amount of mixture is required. However, when less cylinder pressure (in effect, less torque) is required, a fully homogeneous charge that occupies the whole combustion chamber is neither required nor desired. Because the fuel needed can be injected at the most advantageous time in the induction/compression stroke, it can be ignited before it's had a chance to fully diffuse across the chamber.

In E-TEC engines, a shaped depression in the piston crown directs the jet of fuel back up toward the centre of the chamber and helps keep it concentrated in the centre of the chamber. Effectively, it acts like a stratified charge engine without the need for a separate combustion chamber. The concentrated nature of the air/fuel charge means that the flame front doesn't have to travel as far to fully consume all of the fuel in the chamber. This means the burn is of shorter duration and this, in turn, allows greater flexibility in ignition timing. Also, there's little, or no fuel against the walls of the chamber or down the sides of the piston (above the top ring) where combustion is quenched by the close proximity of the relatively cool metal surfaces. This, too, reduces HC emissions.

The most dangerous time for CO emissions is at idle when there's no movement to help dissipate the gas from around a boat. Anything that reduces the concentration of this gas is a potential life saver.

Dawson further pointed out that because the air passing through the cylinder during blow-down contains no fuel, CO emissions are much lower in a direct injection two-stroke engine. Also, the stratified nature of the fuel/air mixture at lower engine speeds means that not all of the air in the chamber is exposed to the combustion event and this greatly reduces CO (and NOx) production.

A common plenum-style intake manifold and Helmholtz resonator tuned chambers in the silencer also reduce noise and the inside of the engine cover is lined with specially moulded foam. There's also foam inside the tuned chamber mufflers on the exhaust relief outlets, which is the part of the exhaust that discharges above water and operates at low speed.

In the past, as far as both noise and emissions are concerned, the difference between four and two-stroke engines has been nothing short of a chasm, but direct injection systems' design approaches have changed all that. Now, some of the latest generation two-stroke engines are comparable with four-stroke motors. Who'd have thought?