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What: Shares of natural-gas engine-technology expert Westport Innovations (NASDAQ:WPRT) shot up 13% in afternoon trading on December 18. The stock was trading up around 3% for most of the day following a new partnership with Austrian company AVL List, which works closely with OEMs in the design and integration phase of product development.

The two companies will work together to develop and deliver powertrain systems featuring Westport's HPDI natural gas injection technology to vehicle OEMS. This is certainly a positive step forward for Westport in its efforts to gain more widespread adoption of natural gas as an alternative to diesel fuel for medium- and heavy-duty trucking and transportation. 

So what: The big announcement came this afternoon, however, sending shares up another 10%, to a 13% gain for the day. Congress announced that it had (finally) passed legislation to reenact and extend the alternative fuels tax credit for 2015, and extend it for all of 2016 at the same time. 

This is good news for fuel buyers and sellers, because this tax credit is worth $0.50 per gallon equivalent of natural gas, furthering the cost advantage of this almost entirely domestically sourced, lower emissions-producing fuel. For example, natural gas transportation leader Clean Energy Fuels Corp (NASDAQ:CLNE) makes a living selling fuel, and will get a direct benefit from this legislation. The company will likely collect between $25 million and $30 million in tax credits in early 2016 based on its fuel sales for 2015, and a similar amount in early 2017 for its fuel sales next year. That will go a long way toward helping the company cover its capital needs next year as it moves forward in reducing debt and reducing its operating costs. 

It's not as clear how much it will benefit Westport, as the company won't directly get any tax benefit. The upside is that it's an additional incentive for truck buyers to consider natural gas; but whether it puts enough "on-the-fence" buyers over the top or not will be debatable. 

Now what: This is good news, but it's also a reminder of how short Mister Market's memory is. The industry goes through this same dance with Congress every couple of years. The tax credits have been in effect for more than seven years now, but about half the time, they were enacted after the fact and made retroactive. They have never been in effect for more than about one year at a time. 

The market hates uncertainty, and so do fleet operators, who see fuel costs as one of their biggest line items. It's nice having this back on the books, but it would be much better if Washington could get some long-term legislation in place. 

In summary, it's good news, but not really "needle moving" for Westport. It's probably a bigger boost to fuel sellers like Clean Energy, and to the customers who buy the fuel from them.

Jason Hall owns shares of Clean Energy Fuels and Westport Innovations. Jason Hall has the following options: long January 2017 $3 calls on Westport Innovations, long January 2017 $5 calls on Clean Energy Fuels, and short January 2017 $5 puts on Clean Energy Fuels. The Motley Fool recommends Clean Energy Fuels and Westport Innovations. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.