Emissions Tuning Fundamentals: Ambient Conditions

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Ambient Conditions

09.10

00:00	When it comes to unwanted engine emissions, there are many potential causes.
00:04	I'll get into the specifics on combustion later but first I'm going to discuss some factors impacting emissions that are completely unrelated to the tune or vehicle modifications.
00:14	In any testing and development process, any time you can reduce the number of variables and outside influences, you can improve the quality of your data and can make better use of it.
00:25	Some of the factors I'll mention may seem like they wouldn't have a large impact but in the world of clean burning modern vehicle emissions, we're typically dealing with very low limits and it all matters.
00:35	For example, when a vehicle is emitting 0.002 grams per mile of non methane hydrocarbons, it doesn't take much to skew that result.
00:45	In this module we'll focus on ambient conditions.
00:48	These are factors you can't fully control but I can give you ways to plan and work with them.
00:54	You can't control the weather unless you're working in a climate controlled chamber but you can keep an eye on the weather reports and trends and make the best of the situation.
01:03	Let's explain a little further.
01:05	By looking at a multi day forecast, perhaps you'll find every morning at 8am that the temperature is within a couple of degree window at a given time of year and humidity is also relatively consistent at that time of the day.
01:18	In your area, perhaps it's 2-4pm that's more consistent day to day.
01:22	So check your local trends to see when ambient conditions are most consistent and repeatable.
01:28	If you see there's a significant storm forecast or an unseasonably cold or hot day, you can take a step back and decide that that's not an ideal day to try calibration or part changes, especially if the change is likely to be a small one.
01:42	If ambient conditions aren't favourable on a given day, you can take it as an opportunity to purposely test how that external change impacts your engine.
01:51	In that case, you'd stick with the same engine calibration and parts so that you can see how the weather changes affect the vehicle behaviour.
02:00	Let's now run through how key ambient factors impact emissions, starting with air temperature, where the first and biggest impact comes from affecting engine, fluid and component starting temperatures.
02:12	If a vehicle has been sitting long enough, the engine itself, coolant, oils, catalysts and other emissions and drivetrain components have all settled at or near to the ambient air temperature.
02:25	In the emissions world, this is called a cold soak.
02:28	Doesn't necessarily mean it's cold outside, just that the ambient air has had sufficient time to bring vehicle components down to the ambient temperature.
02:37	The colder the ambient air temperature the vehicle is soaked to, the more additional fuel enrichment is required for start up and warm up.
02:45	To understand this, let's consider what happens when the engine is at normal operating temperature.
02:51	In this condition, the intake port, valves and intake manifold are all hot and when the fuel is delivered by the injector, it contacts these hot components and goes through a phase change from liquid to vapour.
03:02	This is known as vapourisation and fuel in vapour form is very easy to combust.
03:07	When these components are cold on the other hand, some of the injected fuel will tend to condense and wet out on the port wall and enter the combustion chamber in a liquid form which is more difficult to combust.
03:18	When vapourisation is less complete like this, fewer molecules of fuel are available for combustion.
03:25	Delivering an appropriate excess of fuel ensures there's enough molecules available to combust with the oxygen and generates sufficient torque for the engine to idle smoothly.
03:35	As heat builds, more fuel vapourises, the engine operates more efficiently and less excess fuel delivery is required.
03:42	In the meantime, unburned hydrocarbons exit the exhaust in gaseous, liquid and solid form and incomplete combustion has other consequences we'll discuss later.
03:52	The other ambient factors we'll mention next are worth noting because they can impact the engine load required to achieve or maintain vehicle speed and anything that causes an increase in engine load beyond a certain level may also increase emissions output.
04:07	This is going to be a common theme throughout this course so it's important to clarify that increased load doesn't always mean increased emissions.
04:16	That said, many gas engines have a point beyond which component protection is required, often in the form of additional fuel enrichment to reduce the temperature of combustion.
04:27	On a diesel, supporting additional load often means delivering more fuel to create additional torque.
04:32	As factors that increase engine load stack up, you may transition into a range where emissions significantly increase and that's something we want to avoid except when absolutely necessary.
04:44	First, colder, more dense air creates more aerodynamic drag on the exterior of the vehicle so the engine has to increase output to achieve or maintain a given road speed.
04:55	Colder drivetrain fluids also increase resistance to motion significantly.
05:00	We'd suggest taking readings of ambient temperature, pressure and humidity as well as vehicle component and fluid temperatures at the start, end and throughout emissions tuning development.
05:12	The emissions impacts of ambient conditions will vary a little by application but decreasing barometric pressure generally increases emissions.
05:21	Diesel and gas engines are both impacted by this with diesels being more heavily affected.
05:25	Those testing at high altitude will generally find that their data can't compete with data recorded near sea level but that doesn't mean that you can't still make progress.
05:35	If you run a test, make a change, then get better results with the same operating and ambient conditions, you've made an improvement.
05:43	High altitude testing might not ensure an optimal sea level result but don't get discouraged.
05:49	Humidity is another factor but before I explain how it affects the vehicle, let's clear something up.
05:54	When you see a weather report, they generally tell you the relative humidity but skip the keyword relative and just say currently it's 50% humidity.
06:04	Relative humidity refers to the percentage of water in the air compared to the maximum amount of water the air can hold at a given temperature.
06:12	The potential for air to hold water increases dramatically at hotter air temperatures.
06:18	For example air at 50% relative humidity at 20°C is holding about twice as much water as 50% relative humidity at 10°C.
06:27	If you're involved in or watch drag racing, you may be familiar with an absolute measurement of humidity referred to as water grains.
06:35	There are 7000 grains of water in 1 pound of water.
06:38	To give you a ballpark, 20-200 grains of water per pound of air is a range you might encounter and CARB wants either 50 or 100 water grains per pound of dry air at 20-30°C for their common aftermarket part tests.
06:53	If you'd like to review more test procedure criteria, we've linked to some in the module notes.
06:59	It would appear that CARB understands that maintaining 20-30°C in a test cell is doable but maintaining 50 grains of water for the duration of vehicle testing isn't necessarily realistic.
07:12	For that reason they rely on lab grade equipment to correct emissions values dynamically as the monitored humidity level varies during testing.
07:20	It sheds some light on how you can expect your NOx readings to shift as humidity changes.
07:26	While intake air temperature, barometric pressure and intake manifold pressure are all commonly sensed and available to engine computers, humidity is not.
07:36	This presents a gap in both monitoring and calibration optimisation so recognise this is an area where you'll want to take your own readings or at least check your local weather report.
07:46	Humidity tends to be lower at high altitude but because air molecules are further apart, the water can have a more disruptive effect and shouldn't be discounted entirely.
07:57	At lower altitudes and higher temperatures, high humidity can have dramatic effects.
08:02	The last ambient air condition we'll discuss is wind and if you're road testing for fuel economy, wind can be particularly disruptive.
08:10	A strong wind can increase or reduce the engine output required to achieve a given speed and engine output is often tied to fuel consumption and emissions output.
08:20	If you're driving into a headwind, taking measurements in both directions will give you a feel for the impact the wind is having.
08:27	Now let's recap the major ambient factors.
08:30	These are air temperature, air pressure, humidity and wind.
08:34	Air temperature affects the cold starting temperature of the engine and fluids and plays a role in air density.
08:41	Humidity affects engine operation but engines usually don't have humidity sensors to monitor and correct for its effects.
08:48	While air temperature and humidity may vary significantly each day, you can achieve a level of consistency by testing at times of the day when these conditions remain similar.
08:58	Temperature and humidity tend to follow a daily pattern with humidity typically lowest around midday to afternoon and highest in the morning and evening.