Cold Start 101

By Ron Kotrba | February 05, 2008
When the Brazilian government set out in the 1980s to substitute ethanol for gasoline as its national passenger transportation fuel, General Motors Corp. was acutely aware of the low-volatility and cold-start concerns associated with running an engine on pure alcohol.

According to Henrique Pereira with GM Powertrain, this is why the Big Three automaker equipped its Brazilian vehicles with two fuel tanksone for ethanol or ethanol blends, and the other solely for gasoline to assist with cold starts. While the words "cold" and "Brazil" aren't often found in the same sentence, temperatures below 64 degrees Fahrenheit may cause trouble for an engine starting on alcohol. Ethanol in pure form and in higher blends with gasoline doesn't evaporate well at lower temperatures and won't produce a combustible mix at start-up. However, low concentrations of ethanol in gas, especially at 6 percent, cause inordinately high volatility compared with straight gasoline or ethanol blended in concentrations greater than 20 percent (see chart). Even though this is still a concern regarding emissions (evaporative and permeation), high-volatility E10 burned in older carbureted engines had a propensity to cause vapor lock, which has been much less of a concern in the past 20 years with the advent and proliferation of fuel-injection systems.

Like many fuels, the percentage of ethanol in E85 is adjusted throughout the seasons. During colder months, the ethanol content in E85 drops to nearly 70 percent to compensate for ethanol's lower volatility, and it increases during the warmer months of the year when cold starts aren't a concern. Brazilian vehicles weren't flexible-fuel vehicles (FFVs) at first but were designed for ethanol-based fuels, which is why compression ratios were increased to optimize thermal efficiency in ethanol combustion. Over time, improvements were made, and in 2006, Honda announced it was introducing an FFV able to run on E20 or pure ethanol. Much like GM's early Brazilian ethanol cars, a Honda press release states, "[A] cold-start system utilizing a secondary fuel tank ensures reliable starts even at low ambient temperatures," which indicates that after two decades, automakers still rely on an auxiliary fuel tank to overcome low-volatility issues with alcohol fuels.

Source: SAE Paper 852116

In the 1990s, changing emissions regulations in Brazil and improved combustion technologies led GM to incorporate electronic controls to assist cold-start performance with a greater sense of sophistication. According to GM, the first FFV engines used a light-reactive sensor to measure the composition of fuel relative to alcohol content. In the past few years, GM has switched to what's called a virtual sensor, which functions on readings from the oxygen sensors in the exhaust stream, fuel-level sensor and vehicle-speed sensors. Using these readings, the engine control module adjusts the length of time that fuel injectors stay open depending on the concentration of ethanol. In other words, the more ethanol (and lower energy density), the longer the injectors stay open.

In April 2007, GM issued a large voluntary service campaign for 2006 and 2007 FFVs, which were prone to poor cold-start performances due to improper programming in the engine control module. Ford Motor Co. had a similar issue with its Taurus FFVs (see January 2007 EPM Flex Factor). Similar to how carbureted engines were sometimes equipped with a manual choke to create a fuel-rich air-fuel ratio, the engine needs more fuel and less air at start-up to help combat poor cold-starting with ethanol (without an additional fuel tank for gasoline only).

Change2E85, a company that makes aftermarket E85 conversion kits called FFI Platinum, recently announced that its systems will now come equipped with what it calls "Cold Start Technology." According to the company, "Ethanol has a vaporization temperature approximately 56 degrees Fahrenheit. When the outside temperature drops below 56, the FFI Platinum's cold-start feature will turn on but only during the initial starting cycle. It will add extra fuel during the starting cycle to help increase the vaporization potential. The FFI Platinum will resume normal operations after 30 to 40 seconds of engine run time."