If you've looked for a way to invest in the growth of natural gas vehicles, chances are you've looked at companies like Westport Innovations and Clean Energy Fuels, or Westport partner and leading diesel engine maker Cummins. However, the three companies that are probably doing more for natural gas vehicles than the rest aren't likely to be the first to come to mind: United Parcel Service (NYSE:UPS)Waste Management (NYSE:WM), and General Electric (NYSE:GE)

Let's take a closer look at what these three giants of their industries are doing, and how their steps go beyond just advancing the industry and add real value to their businesses. 

Leveraging its assets 
Waste Management is the largest waste removal company in the U.S. and has been at the forefront of green initiatives for years. What's quite compelling about Waste Management's renewable practices is that they are as much about how they can help the bottom line as they are about sustainability. Already, Waste Management generates enough biogas to power 440,000 homes, and in California's Bay Area the company fuels more than 300 garbage trucks from biomethane produced from its Altamont, California, landfill:

Source: Waste Management.

Waste Management operates 50 natural gas fueling stations as well, working with companies such as Clean Energy Fuels to construct and maintain the stations, so the company can focus on its core business. Eighteen of the locations are publicly accessible, but the idea isn't to go into another business: The stations have to provide an economic benefit to Waste Management, and public access cannot interfere with its daily operations. 

How big is the company's initiative around natural gas vehicles? As of February, only 2,600 of the company's 18,000 vehicles ran on natural gas, but the company plans to have 80% of its fleet converted to NG by 2020. That leaves another roughly 12,000 vehicles to replace. Considering that management expects this initiative to be worth $1 billion in savings, I'm betting they get there. 

However, everything's not all roses with the company's green efforts. In 2013, it took a $483 million impairment charge related to its waste-to-energy business. The good news is this is a non-cash charge. The bad news? Goodwill starts out based on real cash investments, and it took almost 20 years for the company to rationalize this part of goodwill.

Here's a look at three important metrics for Waste Management:

WM Dividend Chart

WM Dividend data by YCharts

The company is using its massive cash flow to buy back shares, increase its dividend, and invest in improving the business and lowering costs. Sustainable initiatives are a big part of this process.

GE's LM2500 Fast Ferry turbine engine is designed for LNG. Source: GE.

Involved in every part of the value chain
General Electric is a diversified industrial giant, with ties to every part of the natural gas story. Not only does GE make key technology like liquefaction systems and natural gas compressors that are used to produce NG for vehicles, but it also is one of the largest locomotive builders. GE is working with BNSF Railways, CSX, and others to develop natural gas conversion systems for existing diesel locomotives, and new LNG-fueled locomotives. The company's NG engines also work in several maritime applications, which are attractive targets for low-cost and cleaner LNG. 

Its diversity in the utility (gas turbines for electric plants), oil and gas (involvement in almost every aspect of production, exploration, and distribution), and transportation segments puts GE more at the middle of the story than any other company. 

When evaluating GE, remember it has two parts of its business: finance and industrial. Over the next several years, the company intends to continue to spin off and divest parts of its finance business in an effort to streamline the organization, reduce exposure to consumer debt, and become more focused on its industrial roots.

For investors, it may seem like a reduction of exposure to a very profitable business, but to draw that conclusion ignores the hard lesson learned during the financial crisis, when GE was nearly crippled from the weight of its consumer-finance exposure. A less complex, more focused industrial business will be a better long-term GE than the conglomerate it was becoming. 

Almost 20% of UPS's heavy trucks will be LNG by this fall. Source: UPS.

What can Brown do with blue?
United Parcel Service operates one of the largest vehicle fleets in the world, consisting of over 100,000 vehicles. Of these, 3,152 run on alternative fuel, everything from natural gas to propane to electric. The company's fleet of heavy trucks totals 5,700 units. In January, UPS announced that it will have between 900 and 1,000 LNG-powered trucks on the road by August. UPS is investing more than $50 million to build enough private LNG stations to -- combined with public stations over the road -- support even more LNG tractors. 

But it doesn't stop there for UPS, which operates more than 900 CNG-powered delivery vans and is adding 1,000 propane-powered vans. As UPS's Kelly Pruitt said in this recent video, "Our strategy has always been to look for technologies that make financial sense, as well as have a significant environmental impact."

But the company's efforts on LNG for trucking are really at the heart. After all, the total number of alternative fuel vehicles makes up only about 3% of UPS's fleet, but LNG-powered tractors will make up almost 20% of the company's heavy trucks. Here's another metric to show how substantial UPS's commitment to natural gas for heavy trucks is: The LNG trucks UPS receives in 2014 could make up as much as 15% of the total number of natural gas-powered heavy trucks sold in the United States. 

Final thoughts: Sustainable practices help enhance returns 
All three companies have kept free cash flow steady or grown it since the financial crisis, while buying back shares at the same time:

WM Free Cash Flow (TTM) Chart

WM Free Cash Flow (TTM) data by YCharts

The result? Share price and dividend growth:

UPS Chart

UPS data by YCharts

These efforts in NGVs go beyond being sustainable practices that are good for the planet -- they also add real, sustainable returns to the bottom line.