The Truth About Cars » andrew bell http://www.thetruthaboutcars.com The Truth About Cars is dedicated to providing candid, unbiased automobile reviews and the latest in auto industry news. Sun, 27 Jul 2014 20:45:49 +0000 en-US hourly 1 http://wordpress.org/?v=3.9.1 The Truth About Cars is dedicated to providing candid, unbiased automobile reviews and the latest in auto industry news. The Truth About Cars no The Truth About Cars editors@ttac.com editors@ttac.com (The Truth About Cars) 2006-2009 The Truth About Cars The Truth About Cars is dedicated to providing candid, unbiased automobile reviews and the latest in auto industry news. The Truth About Cars » andrew bell http://www.thetruthaboutcars.com/wp-content/themes/ttac-theme/images/logo.gif http://www.thetruthaboutcars.com Ask An Engineer: Natural Gas For Dummies http://www.thetruthaboutcars.com/2012/06/ask-an-engineer-natural-gas-for-dummies/ http://www.thetruthaboutcars.com/2012/06/ask-an-engineer-natural-gas-for-dummies/#comments Wed, 27 Jun 2012 15:49:48 +0000 http://www.thetruthaboutcars.com/?p=450553

Westport Innovations has just signed a second deal with General Motors to produce light duty natural gas engines, and it’s probably not the last time we’ll be seeing these kind of partnerships forming. Natural gas vehicles have been explored previously on TTAC, but the technology hasn’t been fully explored in-depth, aside from some well-informed comments in various articles.

As a fuel for vehicles (light duty as well as commercial vehicles), natural gas has a number of attributes which fit well with our current political narratives and economic realities

  1. Natural gas is 30-50% cheaper than diesel per unit of energy
  2. Abundant domestic supply
  3. Environmental benefits (lower GHG and tailpipe emissions)
  4. Significant reduction in CO2, CO, UHC, NOx, SOx and PM emissions versus conventional gasoline and diesel engines.

Natural gas can be used across the full spectrum of spark ignition (gasoline type) and compression ignition (diesel type) engines with the appropriate enabling technologies. While spark ignition natural gas engines have been available for quite some time (such as the NG powered Honda Civic), compression ignition natural gas engines have required further development. The difficulty is that while natural gas burns cleanly, it is less likely to auto-ignite (octane rating of 120-130), unlike diesel, which has a lower octane number. This quality of natural gas is advantageous for a spark ignition engine as it prevents detonation and allows for higher compression ratios, but makes it detrimental for a compression ignition engine.

Westport has devised a dual-fuel direct injection system to enable natural gas substitution in a compression ignition engine. The fuel injector at the heart of this system is able to inject both liquid diesel and gaseous natural gas in precisely metered quantities directly into the cylinder. In this system, the diesel fuel ignites as a result of compression as it would in a regular diesel engine. The combusting diesel fuel initiates the natural gas combustion. 93-95% diesel substitution is achievable according to public documentation. This innovation is directed at the heavy-duty diesel market which includes everything from transport trucks to locomotives.

One of the main criticisms is the lack of infrastructure surrounding natural gas. Compressed natural gas (CNG) is easier to store and transport than liquefied natural gas (LNG) so it is the optimal choice for light duty applications. LNG has a greater volumetric energy density but is more expensive to store, transport and ultimately use in a vehicle as it must be kept cold and pressurized to remain a liquid.

Vehicles like the Civic Natural Gas have a reduced range relative to a gasoline Civic, but commercial vehicles, like transport trucks, are emerging as one of the prime candidates for natural gas engines. Large transport trucks are a significant contributor to green house gas emissions and are on the road enough to make the conversion cost effective – though LNG, rather than CNG, would be the fuel of choice. A relatively small number of LNG filling stations placed along major transport corridors could meet their fueling needs and present a great way to thoroughly evaluate the technology. Less complex CNG stations could be added if the decision was made to target light duty vehicles.

Going “all in” on CNG/LNG is a little premature at this point, but the adoption of natural gas as a transport fuel is a good first step in reducing our emissions while other alternative technologies reach maturity. More in-depth discussion is always welcome in the comments.

“Ask an Engineer” is hosted by Andrew Bell, a mechanical engineer and car enthusiast. Andrew has his MASc in Mechanical Engineering from the University of Toronto, and has worked on Formula SAE teams, as well as alternative fuel technologies in Denmark and Canada. Andrew’s column will explore engineering topics in the most accessible manner possible.

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Ask An Engineer: GDI Problems In A Nutshell http://www.thetruthaboutcars.com/2012/05/ask-an-engineer-gdi-problems-in-a-nutshell/ http://www.thetruthaboutcars.com/2012/05/ask-an-engineer-gdi-problems-in-a-nutshell/#comments Thu, 03 May 2012 13:00:11 +0000 http://www.thetruthaboutcars.com/?p=442513

“Ask an Engineer” is hosted by Andrew Bell, a mechanical engineer and car enthusiast. Andrew has his MASc in Mechanical Engineering from the University of Toronto, and has worked on Formula SAE teams, as well as alternative fuel technologies in Denmark and Canada. Andrew’s column will explore engineering topics in the most accessible manner possible.

Even though every other car nowadays seems to offer gasoline direct injection (GDI), Mercedes-Benz was the first to exploit this technology in the 1955 300SL. But it wasn’t until the mid-1990’s that other automakers started to use GDI in mass produced vehicles. GDI promises marginal increases in fuel economy (3% reduction in BSFC ) but its real benefits include reduced cold start/low load emissions and higher power outputs. While the technology offers engineers incredible flexibility from an engine design perspective, it is not without faults. As with any new technology it is important to understand both the positives and negatives before you choose, say a compact car with GDI or one regular fuel injection. If you want to keep your car for a long period of time, the long-term reliability of a GDI engine is an important factor.

The effect of increased percentages of ethanol on injector longevity.

The percentage of ethanol in gasoline at the pumps is steadily increasing. Ethanol has a tendency to increase the corrosion rate of the various metals used in an engine. Add this to the elevated fuel pressure and the fact the injector is directly exposed to in-cylinder combustion events, and you have a recipe for a recall. Furthermore, these injectors are very sensitive to fuel quality due to outrageously tight tolerances. It is very important to use high quality fuels and keep the filters clean.

Higher pressures in general.

GDI requires significantly higher fuel inlet pressures than port injection. This puts a great deal of strain on every piece of the fuel delivery chain. This is not a problem on a new engine. 50,000 miles down the road, and it may be. Manufacturers have been relatively proactive in this department by specifying robust, stainless steel fuel lines and connections. That hasn’t stopped fuel pump recalls from already occurring

Carbon buildup on intake valves.

This is the big problem with most current GDI engines. Due to modern unburned hydrocarbon (UHC) regulations, vapors from the crankcase are usually vented into the intake stream in order to prevent oil droplets from escaping through the exhaust. In a port injection engine, these droplets are ‘washed off’ the neck of the intake valve by a relatively constant stream of gasoline droplets. In a GDI engine, the gasoline doesn’t touch intake side of the valve. As a result, the droplets have a tendency to bake onto the valve and significantly reduce performance. To add to this effect, many advanced GDI engines also include exhaust gas recirculation in order to lean out the combustion mixture and reduce in-cylinder temperatures for certain combustion modes (reducing NOx emissions). Since GDI combustion has the ability to produce far more soot than premixed combustion (port injection), the problem is magnified.

Even more alarming is that these deposits can dislodge and damage other downstream components (turbochargers, catalytic converters, etc.). Manufacturers have added systems to capture these oil droplets and particulates, but no system is 100% effective. As a result, there are many disappointed early adopters with large repair bills. Even diesel engines haven’t been immune to these issues.

The reason these issues have slipped through to production is that they won’t show up in a 500,000 mile torture test. These types of issues will appear after years of short trips (preventing the engine from reaching operating temperature), bad batches of fuel, etc. As we approach the efficiency limits of the internal combustion engine, the engines themselves (and associated support systems) have become more complex. As with the transition from carburetors to electronic fuel injection, there will be some overlap between relatively bombproof port injected engines and the unproven, first-generation GDI engines.

 

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